The Progressive Corporation (PGR) 10-Q2 Nov 212021 Q3 Quarterly reportFinancial data

Company Profile

Document and Entity Information

Document and Entity Information	9 Months Ended
Document and Entity Information	Sep. 30, 2021shares
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2021
Document Transition Report	false
Entity File Number	001-09518
Entity Incorporation, State or Country Code	OH
Entity Tax Identification Number	34-0963169
Entity Address, Address Line One	6300 Wilson Mills Road,
Entity Address, City or Town	Mayfield Village,
Entity Address, State or Province	OH
Entity Address, Postal Zip Code	44143
City Area Code	440
Local Phone Number	461-5000
Entity Information, Former Legal or Registered Name	Not Applicable
Title of 12(b) Security	Common Shares, $1.00 Par Value
Trading Symbol	PGR
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding	585,063,639
Entity Registrant Name	PROGRESSIVE CORP/OH/
Entity Central Index Key	0000080661
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Amendment Flag	false

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues
Net premiums earned	$ 11,364.8	$ 9,973.5	$ 32,767.3	$ 29,052.8
Investment income	208.9	230.5	639.8	715.5
Net realized gains (losses) on securities:
Net realized gains (losses) on security sales	53.9	195.9	607.8	771.1
Net holding period gains (losses) on securities	(15.9)	336.7	479.8	98.7
Net impairment losses recognized in earnings	(1.2)	0	(3.7)	0
Total net realized gains (losses) on securities	36.8	532.6	1,083.9	869.8
Fees and other revenues	174.9	151.8	516.8	434.8
Service revenues	73.8	58.8	202.1	169.4
Total revenues	11,859.2	10,947.2	35,209.9	31,242.3
Expenses
Losses and loss adjustment expenses	9,250.7	6,713.1	24,767.6	18,189.7
Policy acquisition costs	951.5	835.2	2,754.7	2,413.5
Other underwriting expenses	1,384.4	1,330.9	4,306	4,179.7
Policyholder credit expense	0	29	0	1,062.4
Investment expenses	6.6	4.7	18.5	14.5
Service expenses	72.8	54.4	190	154.7
Interest expense	54.2	56.4	167	160.8
Total expenses	11,720.2	9,023.7	32,203.8	26,175.3
Net Income
Income before income taxes	139	1,923.5	3,006.1	5,067
Provision for income taxes	20.5	392.7	617.5	1,046.7
Net income	118.5	1,530.8	2,388.6	4,020.3
Other Comprehensive Income (Loss)
Total net unrealized gains (losses) on fixed-maturity securities	(129.8)	(24.6)	(578.3)	605.6
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax	0.1	0.2	0.6	0.6
Foreign currency translation adjustment	(0.2)	0	(0.7)	0
Other comprehensive income (loss)	(129.9)	(24.4)	(578.4)	606.2
Comprehensive income (loss)	(11.4)	1,506.4	1,810.2	4,626.5
Computation of Earnings Per Common Share
Net income	118.5	1,530.8	2,388.6	4,020.3
Less: Preferred share dividends	6.7	6.7	20.1	20.1
Net income available to common shareholders	$ 111.8	$ 1,524.1	$ 2,368.5	$ 4,000.2
Average common shares outstanding - Basic	584.7	585.1	584.7	584.9
Net effect of dilutive stock-based compensation	2.4	2.7	2.6	2.8
Total average equivalent common shares - Diluted	587.1	587.8	587.3	587.7
Basic: Earnings per common share	$ 0.19	$ 2.60	$ 4.05	$ 6.84
Diluted: Earnings per common share	$ 0.19	$ 2.59	$ 4.03	$ 6.81

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Assets
Fixed maturities (amortized cost: $44,556.0, $35,082.8, and $35,589.1)		$ 45,045.7	$ 36,810.9	$ 36,313.5
Short-term investments (amortized cost: $1,088.7, $4,667.8, and $5,218.5)		1,088.7	5,218.5	4,667.8
Total available-for-sale securities		46,134.4	42,029.4	40,981.3
Nonredeemable preferred stocks (cost: $1,479.9, $1,302.7, and $1,358.7)		1,572.8	1,447.9	1,323.2
Common equities (cost: $1,238.4, $1,130.0, and $1,187.3)		4,580.2	4,053	3,459.8
Total equity securities		6,153	5,500.9	4,783
Total investments		52,287.4	47,530.3	45,764.3
Cash and cash equivalents		270.6	76.5	90.1
Restricted Cash and Cash Equivalents		14.9	0	1
Cash and cash equivalents		285.5	76.5	91.1
Accrued investment income		168.6	176.4	181.5
Premiums receivable, net of allowance for credit losses of $257.3, $313.2, and $356.2		10,246.7	8,160.1	8,574.3
Reinsurance recoverables		5,044.9	4,019.4	3,987.3
Prepaid reinsurance premiums		661.1	368.1	463.4
Deferred acquisition costs		1,430.7	1,237.2	1,265
Property and equipment, net of accumulated depreciation of $1,422.2, $1,264.5, and $1,291.4		1,161	1,106	1,181.6
Goodwill		452.7	452.7	452.7
Intangible assets, net of accumulated amortization of $369.2, $312.0, and $326.1		131.9	171.4	185.5
Other assets		747.6	800.2	759.3
Total assets		72,618.1	64,098.3	62,906
Liabilities
Unearned premiums		16,671.4	13,437.5	14,199.3
Loss and loss adjustment expense reserves		25,926.3	20,265.8	19,589.2
Net federal deferred income taxes		110.7	310	227.9
Dividends payable on common shares		58.5	2,694.5	58.6
Accounts payable, accrued expenses, and other liabilities		6,394.9	4,955.8	5,348.4
Debt	[1]	4,898.2	5,396.1	5,395.4
Total liabilities		54,060	47,059.7	44,818.8
Shareholders’ Equity
Serial Preferred Shares, Series B, no par value (cumulative, liquidation preference $1,000 per share) (authorized, issued, and outstanding 0.5)		493.9	493.9	493.9
Common shares, $1.00 par value (authorized 900.0; issued 797.6, 797.5, and 797.5, including treasury shares of 212.6, 211.9, and 212.3)		585	585.2	585.6
Paid-in capital		1,735.9	1,672.9	1,629.4
Retained earnings		15,390	13,354.9	14,427.7
Accumulated other comprehensive income (loss):
Net unrealized gains (losses) on fixed-maturity securities		369	947.3	966.4
AOCI, Cash Flow Hedge, Cumulative Gain (Loss), after Tax		(15)	(15.6)	(15.8)
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax		(0.7)	0	0
Total accumulated other comprehensive income (loss)		353.3	931.7	950.6
Total shareholders’ equity		18,558.1	17,038.6	18,087.2
Total liabilities and shareholders’ equity		$ 72,618.1	$ 64,098.3	$ 62,906

[1]	Consists of long-term debt. See Note 4 – Debt for further discussion.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Fixed maturities, amortized cost	$ 44,556	$ 35,589.1	$ 35,082.8
Short-term investments, amortized cost	1,088.7	5,218.5	4,667.8
Nonredeemable preferred stocks, cost	1,479.9	1,358.7	1,302.7
Common equities, cost	1,238.4	1,187.3	1,130
Allowance for credit loss, beginning of period	257.3	356.2	313.2
Property and equipment, accumulated depreciation	1,422.2	1,291.4	1,264.5
Intangible Assets, Accumulated Amortization	$ 369.2	$ 326.1	$ 312
Common Shares, par value (USD per share)	$ 1	$ 1	$ 1
Common Shares, authorized (shares)	900,000,000	900,000,000	900,000,000
Common Shares, issued (shares)	797,600,000	797,500,000	797,500,000
Common Shares, treasury shares (shares)	212,600,000	212,300,000	211,900,000
Serial Preferred Stock
Preferred Stock, Shares Authorized (shares)	20,000,000	20,000,000	20,000,000
Series B Preferred Stock \| Serial Preferred Stock
Preferred Stock, No Par Value	$ 0	$ 0	$ 0
Preferred Stock, Shares Authorized (shares)	500,000	500,000	500,000
Preferred Stock, Shares Issued (shares)	500,000	500,000	500,000
Preferred Stock, Shares Outstanding (shares)	500,000	500,000	500,000
Preferred Stock, Liquidation Preference Per Share (USD per share)	$ 1,000	$ 1,000	$ 1,000

Consolidated Statements of Chan

Consolidated Statements of Changes in Shareholders' Equity Statement - USD ($) $ in Millions	Total	Serial Preferred Stock	Preferred Shares, No Par Value	Common Shares, $1.00 Par Value	Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss) Attributable to Progressive	Series B Preferred StockSerial Preferred Stock	Voting Preference Shares
Common Stock, Dividends, Per Share, Declared	$ 0.30
Preferred Stock, Dividends Per Share, Declared								$ 53.750
Balance, Beginning of period at Dec. 31, 2019			$ 493.9	$ 584.6	$ 1,573.4	$ 10,679.6	$ 341.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 4,020.3					4,020.3
Treasury shares purchased				(0.9)	(2.5)	(71.1)
Cash dividends declared on common shares ($0.10, $0.10, $0.30, and $0.30 per share)						(175.3)
Cash dividends declared on Serial Preferred Shares, Series B ($26.875, $26.875, $26.875, and $53.750 per share)						(26.8)
Reinvested dividends on restricted stock units					1.1	(1.1)
Net restricted equity awards issued/vested				1.9	(1.9)
Amortization of equity-based compensation					61.9
Adjustment to carrying amount of redeemable noncontrolling interest					(2.6)
Attributable to noncontrolling interest							2.7
Other comprehensive income (loss)	606.2						606.2
Other, net						2.1
Balance, End of period at Sep. 30, 2020	$ 18,087.2		493.9	585.6	1,629.4	14,427.7	950.6
Common Stock, Dividends, Per Share, Declared	$ 0.10
Preferred Stock, Dividends Per Share, Declared								26.875
Balance, Beginning of period at Jun. 30, 2020			493.9	585.4	1,614.5	13,001.8	975
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 1,530.8					1,530.8
Treasury shares purchased				(0.5)	(1.5)	(43.4)
Cash dividends declared on common shares ($0.10, $0.10, $0.30, and $0.30 per share)						(58.5)
Cash dividends declared on Serial Preferred Shares, Series B ($26.875, $26.875, $26.875, and $53.750 per share)						(13.4)
Reinvested dividends on restricted stock units					0.4	(0.4)
Net restricted equity awards issued/vested				0.7	(0.7)
Amortization of equity-based compensation					16.7
Adjustment to carrying amount of redeemable noncontrolling interest					0
Attributable to noncontrolling interest							0
Other comprehensive income (loss)	(24.4)						(24.4)
Other, net						10.8
Balance, End of period at Sep. 30, 2020	$ 18,087.2		493.9	585.6	1,629.4	14,427.7	950.6
Preferred Stock, No Par Value								$ 0
Preferred Stock, Shares Authorized (shares)		20,000,000						500,000	5,000,000
Preferred Stock, Shares Issued (shares)								500,000	0
Common Shares, par value (USD per share)	$ 1
Preferred Stock, No Par Value								$ 0
Preferred Stock, Shares Authorized (shares)		20,000,000						500,000
Preferred Stock, Shares Issued (shares)								500,000
Common Shares, par value (USD per share)	1
Common Stock, Dividends, Per Share, Declared	$ 0.30
Preferred Stock, Dividends Per Share, Declared								$ 26.875
Balance, Beginning of period at Dec. 31, 2020	$ 17,038.6		493.9	585.2	1,672.9	13,354.9	931.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	2,388.6					2,388.6
Treasury shares purchased				(1.8)	(5.3)	(160.1)
Cash dividends declared on common shares ($0.10, $0.10, $0.30, and $0.30 per share)						(175.2)
Cash dividends declared on Serial Preferred Shares, Series B ($26.875, $26.875, $26.875, and $53.750 per share)						(13.4)
Reinvested dividends on restricted stock units					1.2	(1.2)
Net restricted equity awards issued/vested				1.6	(1.6)
Amortization of equity-based compensation					68.7
Adjustment to carrying amount of redeemable noncontrolling interest					0
Attributable to noncontrolling interest							0
Other comprehensive income (loss)	(578.4)						(578.4)
Other, net						(3.6)
Balance, End of period at Sep. 30, 2021	$ 18,558.1		493.9	585	1,735.9	15,390	353.3
Common Stock, Dividends, Per Share, Declared	$ 0.10
Preferred Stock, Dividends Per Share, Declared								26.875
Balance, Beginning of period at Jun. 30, 2021			493.9	585.2	1,712.3	15,401	483.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 118.5					118.5
Treasury shares purchased				(0.7)	(2.3)	(68.8)
Cash dividends declared on common shares ($0.10, $0.10, $0.30, and $0.30 per share)						(58.4)
Cash dividends declared on Serial Preferred Shares, Series B ($26.875, $26.875, $26.875, and $53.750 per share)						(13.4)
Reinvested dividends on restricted stock units					0.4	(0.4)
Net restricted equity awards issued/vested				0.5	(0.5)
Amortization of equity-based compensation					26
Adjustment to carrying amount of redeemable noncontrolling interest					0
Attributable to noncontrolling interest							0
Other comprehensive income (loss)	(129.9)						(129.9)
Other, net						11.5
Balance, End of period at Sep. 30, 2021	$ 18,558.1		$ 493.9	$ 585	$ 1,735.9	$ 15,390	$ 353.3
Preferred Stock, No Par Value								$ 0
Preferred Stock, Shares Authorized (shares)		20,000,000						500,000	5,000,000
Preferred Stock, Shares Issued (shares)								500,000	0
Common Shares, par value (USD per share)	$ 1

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020
Statement of Cash Flows [Abstract]
Net income		$ 2,388.6	$ 4,020.3
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation		203.9	201.2
Amortization of intangible assets		43.1	42.8
Net amortization of fixed-income securities		97.2	72.4
Amortization of equity-based compensation		68.7	61.9
Net realized (gains) losses on securities		(1,083.9)	(869.8)
Net (gains) losses on disposition of property and equipment		(4)	4.8
Changes in:
Premiums receivable		(1,994)	(1,067)
Reinsurance recoverables		(573.1)	(608.4)
Prepaid reinsurance premiums		(278.4)	163.1
Deferred acquisition costs		(193.5)	(208.5)
Income taxes		(156.2)	(78.1)
Unearned premiums		3,167	1,810.5
Loss and loss adjustment expense reserves		4,511.7	1,483.8
Accounts payable, accrued expenses, and other liabilities		1,033	319.7
Other, net		79.5	9.6
Net cash provided by operating activities		7,309.6	5,358.3
Purchases:
Fixed maturities		(25,222.2)	(25,974.9)
Equity securities		(464.9)	(781.8)
Sales:
Fixed maturities		11,747.1	18,412.9
Equity securities		625.2	396.2
Maturities, paydowns, calls, and other:
Fixed maturities		5,545.5	5,766.8
Equity securities		119.1	79
Net (purchases) sales of short-term investments		4,209.3	(2,844.1)
Net unsettled security transactions		304.2	457.3
Purchases of property and equipment		(188.3)	(176.4)
Acquisition of Protective Insurance Corporation, net of cash, cash equivalents, and restricted cash acquired	[1]	(313.2)	0
Sales of property and equipment		63.1	13.5
Net cash used in investing activities		(3,575.1)	(4,651.5)
Cash Flows From Financing Activities
Dividends paid to common shareholders		(2,811.5)	(1,492.4)
Dividends paid to preferred shareholders		(26.8)	(26.8)
Acquisition of treasury shares for restricted stock tax liabilities		(66.9)	(67)
Acquisition of treasury shares acquired in open market		(100.3)	(7.5)
Payment of acquired company debt	[1]	(20)	0
Repayments of Long-term Debt		(500)	0
Acquisition of additional shares of ARX Holding Corp.		0	(243)
Net proceeds from debt issuances		0	986.3
Proceeds from exercise of equity options		0	7.3
Net cash used in financing activities		(3,525.5)	(843.1)
Increase (decrease) in cash, cash equivalents, restricted cash, and restricted cash equivalents		209	(136.3)
Cash, cash equivalents, and restricted cash – January 1		76.5	227.4
Cash, cash equivalents, restricted cash, and restricted cash equivalents – September 30		$ 285.5	$ 91.1

[1]	See Note 14 – Acquisition for further discussion.

Basis of Presentation

Basis of Presentation	9 Months Ended
Basis of Presentation	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation — The accompanying consolidated financial statements include the accounts of The Progressive Corporation, our wholly owned insurance and non-insurance subsidiaries and affiliates in which we have a controlling financial interest (Progressive), including, beginning on June 1, 2021, Protective Insurance Corporation and its subsidiaries (Protective Insurance), which Progressive acquired on that date. See Note 14 – Acquisition for further discussion. The consolidated financial statements reflect all normal recurring adjustments that, in the opinion of management, were necessary for a fair statement of the results for the interim periods presented. The results of operations for the period ended September 30, 2021, are not necessarily indicative of the results expected for the full year. These consolidated financial statements and the notes thereto should be read in conjunction with Progressive’s audited financial statements and accompanying notes included in Exhibit 13 to our Annual Report on Form 10-K for the year ended December 31, 2020 (2020 Annual Report to Shareholders). We perform analyses to evaluate our premiums receivables for expected credit losses. See the 2020 Annual Report to Shareholders for a discussion on our premiums receivable allowance for credit loss policy. The following table summarizes changes in our allowance for credit loss exposure on our premiums receivables: Three Months Nine Months Periods Ended September 30, 2021 2020 2021 2020 (millions) Allowance for credit loss, beginning of period $ 245.2 $ 437.9 $ 356.2 $ 283.2 Allowance acquired during period 1 0 0 3.5 0 Increase (decrease) in allowance 2 107.2 (3.6) 255.5 362.9 Write-offs 3 (95.1) (121.1) (357.9) (332.9) Allowance for credit loss, end of period $ 257.3 $ 313.2 $ 257.3 $ 313.2 1 Represents the amount of the allowance acquired in the Protective Insurance acquisition. 2 Represents the incremental increase (decrease) in other underwriting expenses. 3 Represents portion of allowance that is reversed when premiums receivables are written off. For the three months ended September 30, 2021, we experienced an increase in our allowance for credit losses, compared to a decrease during the same period in 2020. Collections received on outstanding premiums receivable balances have increased following the lifting of the moratoriums and billing leniency efforts that we put in place from March through May 2020, which contributed to the decrease in our allowance during the third quarter 2020 and lower allowance recorded for the nine months ended September 30, 2021, compared to the same period in 2020. The greater collections are likely due in part to changes in consumer spending habits and government stimulus spending during the period. Other assets on the consolidated balance sheets include certain long-lived assets that are considered “ held for sale. ” The fair value of these held for sale assets, less the estimated costs to sell, was $12.6 million at September 30, 2021, $27.1 million at September 30, 2020, and $56.6 million at December 31, 2020.

Investments

Investments	9 Months Ended
Investments	Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]
Investments	The following tables present the composition of our investment portfolio by major security type, consistent with our classification of how we manage, monitor, and measure the portfolio. Our securities are reported in our consolidated balance sheets at fair value. The changes in fair value for our fixed-maturity securities (other than hybrid securities) are reported as a component of accumulated other comprehensive income, net of deferred income taxes, in our consolidated balance sheets. The net holding period gains (losses) reported below represent the inception-to-date changes in fair value of the securities. The changes in the net holding period gains (losses) between periods for the hybrid securities and equity securities are recorded as a component of net realized gains (losses) on securities in our consolidated statements of comprehensive income. ($ in millions) Cost Gross Gross Net Fair % of September 30, 2021 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 21,093.1 $ 144.9 $ (95.9) $ 0 $ 21,142.1 40.3 % State and local government obligations 2,123.8 48.8 (6.7) 0 2,165.9 4.1 Corporate debt securities 10,568.7 314.6 (10.5) 1.2 10,874.0 20.8 Residential mortgage-backed securities 624.4 3.4 (0.9) 1.0 627.9 1.2 Commercial mortgage-backed securities 5,739.4 70.2 (20.6) 0 5,789.0 11.1 Other asset-backed securities 4,235.6 29.4 (2.8) 0 4,262.2 8.2 Redeemable preferred stocks 171.0 1.3 (0.7) 13.0 184.6 0.4 Total fixed maturities 44,556.0 612.6 (138.1) 15.2 45,045.7 86.1 Short-term investments 1,088.7 0 0 0 1,088.7 2.1 Total available-for-sale securities 45,644.7 612.6 (138.1) 15.2 46,134.4 88.2 Equity securities: Nonredeemable preferred stocks 1,479.9 0 0 92.9 1,572.8 3.0 Common equities 1,238.4 0 0 3,341.8 4,580.2 8.8 Total equity securities 2,718.3 0 0 3,434.7 6,153.0 11.8 Total portfolio 1 $ 48,363.0 $ 612.6 $ (138.1) $ 3,449.9 $ 52,287.4 100.0 % ($ in millions) Cost Gross Gross Net Fair % of September 30, 2020 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 9,835.1 $ 370.3 $ (1.6) $ 0 $ 10,203.8 22.3 % State and local government obligations 4,382.1 148.2 (0.5) 0 4,529.8 9.9 Corporate debt securities 10,038.7 574.7 (3.6) 2.2 10,612.0 23.2 Residential mortgage-backed securities 525.4 7.0 (1.4) 0 531.0 1.2 Commercial mortgage-backed securities 5,964.4 117.1 (27.9) 0 6,053.6 13.2 Other asset-backed securities 4,155.8 46.8 (2.3) 0 4,200.3 9.2 Redeemable preferred stocks 181.3 2.5 (3.2) 2.4 183.0 0.4 Total fixed maturities 35,082.8 1,266.6 (40.5) 4.6 36,313.5 79.4 Short-term investments 4,667.8 0 0 0 4,667.8 10.2 Total available-for-sale securities 39,750.6 1,266.6 (40.5) 4.6 40,981.3 89.6 Equity securities 2 : Nonredeemable preferred stocks 1,277.7 0 0 20.5 1,298.2 2.8 Common equities 1,155.0 0 0 2,329.8 3,484.8 7.6 Total equity securities 2,432.7 0 0 2,350.3 4,783.0 10.4 Total portfolio 1 $ 42,183.3 $ 1,266.6 $ (40.5) $ 2,354.9 $ 45,764.3 100.0 % ($ in millions) Cost Gross Gross Net Fair % of December 31, 2020 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 12,437.9 $ 305.8 $ (3.7) $ 0 $ 12,740.0 26.8 % State and local government obligations 3,099.4 123.1 (0.7) 0 3,221.8 6.8 Corporate debt securities 9,579.7 601.7 (0.1) 3.9 10,185.2 21.4 Residential mortgage-backed securities 503.3 7.1 (0.9) 0 509.5 1.1 Commercial mortgage-backed securities 6,042.6 142.5 (10.0) 0 6,175.1 13.0 Other asset-backed securities 3,745.0 40.1 (0.5) 0 3,784.6 7.9 Redeemable preferred stocks 181.2 3.6 (1.4) 11.3 194.7 0.4 Total fixed maturities 35,589.1 1,223.9 (17.3) 15.2 36,810.9 77.4 Short-term investments 5,218.5 0 0 0 5,218.5 11.0 Total available-for-sale securities 40,807.6 1,223.9 (17.3) 15.2 42,029.4 88.4 Equity securities 2 : Nonredeemable preferred stocks 1,333.7 0 0 89.2 1,422.9 3.0 Common equities 1,212.3 0 0 2,865.7 4,078.0 8.6 Total equity securities 2,546.0 0 0 2,954.9 5,500.9 11.6 Total portfolio 1 $ 43,353.6 $ 1,223.9 $ (17.3) $ 2,970.1 $ 47,530.3 100.0 % 1 Our portfolio reflects the effect of net unsettled security transactions; at September 30, 2021, we had $399.7 million in other liabilities, compared to $469.2 million and $95.5 million at September 30, 2020 and December 31, 2020, respectively. The total fair value of the portfolio at September 30, 2021 and 2020, and December 31, 2020, included $2.9 billion, $2.8 billion, and $6.2 billion, respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company, net of unsettled security transactions. During the first nine months of 2021, we used a portion of these investments to pay our common share dividends, repurchase common shares, and pay off debt. 2 Includes a $25.0 million reclassification from nonredeemable preferred stocks to common equities to reflect the prior year conversion of a security and to conform to the current year classification. At September 30, 2021, bonds and certificates of deposit in the principal amount of $407.8 million were on deposit to meet state insurance regulatory requirements. We did not hold any securities of any one issuer, excluding U.S. government obligations, with an aggregate cost or fair value exceeding 10% of total shareholders’ equity at September 30, 2021 or 2020, or December 31, 2020. At September 30, 2021, we did not hold any debt securities that were non-income producing during the preceding 12 months and held $12.3 million of foreign government obligations at September 30, 2021, which we classified in our corporate debt portfolio. Short-Term Investments Our short-term investments may include commercial paper and other investments that are expected to mature or are redeemable within one year. We invested in repurchase and reverse repurchase transactions during 2021 and 2020, but did not have any open positions at September 30, 2021 and 2020, or December 31, 2020. To the extent we enter into repurchase or reverse repurchase transactions, consistent with past practice, we would elect not to offset these transactions and would report them on a gross basis on our consolidated balance sheets, despite the option to elect to offset these transactions as long as they were with the same counterparty and subject to an enforceable master netting arrangement. Hybrid Securities Certain securities in our fixed-maturity portfolio are accounted for as hybrid securities because they contain embedded derivatives that are not deemed to be clearly and closely related to the host investments. These securities are reported at fair value: September 30, (millions) 2021 2020 December 31, 2020 Fixed Maturities: Corporate debt securities $ 363.3 $ 101.0 $ 188.4 Residential mortgage-backed securities 217.0 0 0 Other asset-backed securities 104.5 18.7 34.8 Redeemable preferred stocks 133.2 122.5 131.4 Total hybrid securities $ 818.0 $ 242.2 $ 354.6 Since the embedded derivatives (e.g., change-in-control put option, debt-to-equity conversion, or any other feature unrelated to the credit quality or risk of default of the issuer that could impact the amount or timing of our expected future cash flows) do not have observable intrinsic values, we have elected to record the changes in fair value of these securities through income as a component of net realized gains or losses. Fixed Maturities The composition of fixed maturities by maturity at September 30, 2021, was: (millions) Cost Fair Value Less than one year $ 6,124.5 $ 6,153.2 One to five years 27,606.8 27,968.5 Five to ten years 10,761.3 10,861.0 Ten years or greater 63.4 63.0 Total $ 44,556.0 $ 45,045.7 Asset-backed securities are classified in the maturity distribution table based upon their projected cash flows. All other securities, which do not have a single maturity date, are reported based upon expected average maturity. Contractual maturities may differ from expected maturities because the issuers of the securities may have the right to call or prepay obligations. Gross Unrealized Losses The following tables show the composition of gross unrealized losses, for which no credit related impairment has been recognized, by major security type and by the length of time that individual securities have been in a continuous unrealized loss position: Total No. of Sec. Total Gross Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized No. of Sec. Fair Unrealized September 30, 2021 U.S. government obligations 73 $ 13,352.8 $ (95.9) 65 $ 12,152.6 $ (70.1) 8 $ 1,200.2 $ (25.8) State and local government obligations 96 710.3 (6.7) 81 635.9 (4.9) 15 74.4 (1.8) Corporate debt securities 227 1,581.3 (10.5) 225 1,551.5 (10.0) 2 29.8 (0.5) Residential mortgage-backed securities 63 149.3 (0.9) 53 135.1 (0.6) 10 14.2 (0.3) Commercial mortgage-backed securities 70 1,758.8 (20.6) 65 1,543.6 (19.9) 5 215.2 (0.7) Other asset-backed securities 114 1,625.9 (2.8) 109 1,614.2 (2.7) 5 11.7 (0.1) Redeemable preferred stocks 1 11.8 (0.7) 0 0 0 1 11.8 (0.7) Total fixed maturities 644 $ 19,190.2 $ (138.1) 598 $ 17,632.9 $ (108.2) 46 $ 1,557.3 $ (29.9) Total No. of Sec. Total Gross Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized No. of Sec. Fair Unrealized September 30, 2020 U.S. government obligations 13 $ 1,993.8 $ (1.6) 13 $ 1,993.8 $ (1.6) 0 $ 0 $ 0 State and local government obligations 45 320.1 (0.5) 45 320.1 (0.5) 0 0 0 Corporate debt securities 28 537.8 (3.6) 28 537.8 (3.6) 0 0 0 Residential mortgage-backed securities 28 86.7 (1.4) 9 38.7 (1.0) 19 48.0 (0.4) Commercial mortgage-backed securities 77 1,794.8 (27.9) 46 1,170.4 (17.3) 31 624.4 (10.6) Other asset-backed securities 22 191.5 (2.3) 11 146.5 (2.1) 11 45.0 (0.2) Redeemable preferred stocks 1 9.3 (3.2) 0 0 0 1 9.3 (3.2) Total fixed maturities 214 $ 4,934.0 $ (40.5) 152 $ 4,207.3 $ (26.1) 62 $ 726.7 $ (14.4) Total No. of Sec. Total Gross Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized No. of Sec. Fair Unrealized December 31, 2020 U.S. government obligations 9 $ 1,511.0 $ (3.7) 9 $ 1,511.0 $ (3.7) 0 $ 0 $ 0 State and local government obligations 30 208.7 (0.7) 30 208.7 (0.7) 0 0 0 Corporate debt securities 7 129.4 (0.1) 7 129.4 (0.1) 0 0 0 Residential mortgage-backed securities 21 44.4 (0.9) 0 0 0 21 44.4 (0.9) Commercial mortgage-backed securities 43 893.3 (10.0) 9 93.6 (0.3) 34 799.7 (9.7) Other asset-backed securities 22 183.7 (0.5) 9 74.4 (0.1) 13 109.3 (0.4) Redeemable preferred stocks 1 11.0 (1.4) 0 0 0 1 11.0 (1.4) Total fixed maturities 133 $ 2,981.5 $ (17.3) 64 $ 2,017.1 $ (4.9) 69 $ 964.4 $ (12.4) The increase in the number of securities in an unrealized loss position since both September 30, 2020 and December 31, 2020, was primarily the result of an increase in interest rates. As of September 30, 2021, we had two securities in the table above, all in our corporate debt securities, that had their credit ratings downgraded during the quarter, with a combined fair value of $10.5 million and an unrealized loss of $0.4 million. A review of the securities in an unrealized loss position indicated that the issuers were current with respect to their interest obligations and that there was no evidence of deterioration of the current cash flow projections that would indicate we would not receive the remaining principal at maturity. Allowance For Credit and Uncollectible Losses We are required to measure the amount of potential credit losses for all fixed-maturity securities in an unrealized loss position. We did not record any allowances for credit losses or any write-offs for amounts deemed to be uncollectible for the first nine months of 2021 and did not have a material credit loss allowance balance as of September 30, 2021 and 2020, or December 31, 2020. We considered several factors and inputs related to the individual securities as part of this analysis, including: • current performance indicators on the business model or underlying assets (e.g., delinquency rates, foreclosure rates, and default rates); • credit support (via current levels of subordination); • historical credit ratings; and • updated cash flow expectations based upon these performance indicators. In order to determine the amount of credit loss, if any, we initially reviewed securities in a loss position to determine whether it was likely that we would be required, or whether we intended, to sell any of the securities prior to the recovery of their respective cost bases (which could be maturity). If we were likely to, or intended to, sell prior to a potential recovery, we would write off the unrealized loss. For those securities that we determined we were not likely to, or did not intend to, sell prior to a potential recovery, we calculated the net present value (NPV) of the cash flows expected (i.e., expected recovery value) using the current book yield for each security. The NPV was then compared to the security’s current amortized value to determine if a credit loss existed. In the event that the NPV was below the amortized value, and the amount was determined to be material individually, or in aggregate, a credit loss would be deemed to exist, and either an allowance for credit losses would be created, or if an allowance already existed, either a recovery of the previous allowance, or an incremental loss, would be recorded to net realized gains (losses) on securities. As of September 30, 2021, we believe none of the unrealized losses relate to material credit losses on any specific securities, or in the aggregate, based on our review. We continue to expect all the securities in our portfolio to pay their principal and interest obligations. We reviewed our accrued investment income outstanding on those securities in an unrealized loss position at September 30, 2021, to determine if the accrued interest amounts were deemed to be uncollectible. Based on our analysis, we believe the issuers have sufficient liquidity and capital reserves to meet their current interest and future principal, obligations and, therefore, did not write off any accrued income as uncollectible at September 30, 2021. Realized Gains (Losses) The components of net realized gains (losses) for the three and nine months ended September 30, were: Three Months Nine Months (millions) 2021 2020 2021 2020 Gross realized gains on security sales Available-for-sale securities: U.S. government obligations $ 19.6 $ 90.2 $ 97.5 $ 565.8 State and local government obligations 6.0 43.9 50.3 58.9 Corporate and other debt securities 23.6 58.8 84.4 125.5 Residential mortgage-backed securities 0 0 0.3 0 Commercial mortgage-backed securities 1.4 5.0 40.7 15.4 Other asset-backed securities 0.3 0.2 1.0 0.2 Redeemable preferred stocks 0 0 1.5 0 Total available-for-sale securities 50.9 198.1 275.7 765.8 Equity securities: Nonredeemable preferred stocks 2.3 0.3 26.0 20.0 Common equities 6.2 2.0 352.3 77.1 Total equity securities 8.5 2.3 378.3 97.1 Subtotal gross realized gains on security sales 59.4 200.4 654.0 862.9 Gross realized losses on security sales Available-for-sale securities: U.S. government obligations (2.0) (1.5) (28.5) (4.8) State and local government obligations 0 0 (3.0) 0 Corporate and other debt securities (0.7) 0 (6.1) (6.5) Residential mortgage-backed securities (0.3) 0 (0.3) 0 Commercial mortgage-backed securities (0.5) (2.9) (1.6) (12.7) Other asset-backed securities 0 0 (0.4) 0 Total available-for-sale securities (3.5) (4.4) (39.9) (24.0) Equity securities: Nonredeemable preferred stocks (0.3) (0.1) (0.7) (7.5) Common equities (1.7) 0 (5.6) (60.3) Total equity securities (2.0) (0.1) (6.3) (67.8) Subtotal gross realized losses on security sales (5.5) (4.5) (46.2) (91.8) Net realized gains (losses) on security sales Available-for-sale securities: U.S. government obligations 17.6 88.7 69.0 561.0 State and local government obligations 6.0 43.9 47.3 58.9 Corporate and other debt securities 22.9 58.8 78.3 119.0 Residential mortgage-backed securities (0.3) 0 0 0 Commercial mortgage-backed securities 0.9 2.1 39.1 2.7 Other asset-backed securities 0.3 0.2 0.6 0.2 Redeemable preferred stocks 0 0 1.5 0 Total available-for-sale securities 47.4 193.7 235.8 741.8 Equity securities: Nonredeemable preferred stocks 2.0 0.2 25.3 12.5 Common equities 4.5 2.0 346.7 16.8 Total equity securities 6.5 2.2 372.0 29.3 Subtotal net realized gains (losses) on security sales 53.9 195.9 607.8 771.1 Net holding period gains (losses) Hybrid securities 1.0 3.2 0 (3.2) Equity securities (16.9) 333.5 479.8 101.9 Subtotal net holding period gains (losses) (15.9) 336.7 479.8 98.7 Other asset impairment (1.2) 0 (3.7) 0 Total net realized gains (losses) on securities $ 36.8 $ 532.6 $ 1,083.9 $ 869.8 Realized gains (losses) on securities sold are computed using the first-in-first-out method. The significant gains recognized in 2020 on our fixed-income portfolio were the result of selling Treasury securities for opportunistic investments. In 2020, the gains in our common stock portfolio resulted from rebalancing the indexed equity portfolio. For the nine months ended 2021, the realized gain in common equities reflected the sale of a common stock held outside our indexed portfolio. The other asset impairment loss was recorded as a result of our investment in a federal new markets tax credit fund, which was entered into during the second quarter 2021, and reported in other assets in the consolidated balance sheet. The following table reflects our holding period realized gains (losses) on equity securities recognized for the three and nine months ended September 30, 2021 and 2020, for equity securities held at the respective quarter end: Three Months Nine Months (millions) 2021 2020 2021 2020 Total net gains (losses) recognized during the period on equity securities $ (10.4) $ 335.7 $ 851.8 $ 131.2 Less: Net gains (losses) recognized on equity securities sold during the period 6.5 2.2 372.0 29.3 Net holding period gains (losses) recognized during the period on equity securities held at period end $ (16.9) $ 333.5 $ 479.8 $ 101.9 Net Investment Income The components of net investment income for the three and nine months ended September 30, were: Three Months Nine Months (millions) 2021 2020 2021 2020 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 39.0 $ 36.7 $ 106.8 $ 135.0 State and local government obligations 9.7 18.5 34.6 44.2 Corporate debt securities 70.2 76.6 228.6 211.2 Residential mortgage-backed securities 3.2 0.8 9.4 8.8 Commercial mortgage-backed securities 35.0 37.3 105.2 114.2 Other asset-backed securities 16.1 22.6 47.1 76.6 Redeemable preferred stocks 2.3 2.5 7.1 12.5 Total fixed maturities 175.5 195.0 538.8 602.5 Short-term investments 0.4 6.2 2.6 26.9 Total available-for-sale securities 175.9 201.2 541.4 629.4 Equity securities: Nonredeemable preferred stocks 17.3 15.8 52.5 44.3 Common equities 15.7 13.5 45.9 41.8 Total equity securities 33.0 29.3 98.4 86.1 Investment income 208.9 230.5 639.8 715.5 Investment expenses (6.6) (4.7) (18.5) (14.5) Net investment income $ 202.3 $ 225.8 $ 621.3 $ 701.0

Fair Value

Fair Value	9 Months Ended
Fair Value	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Fair Value	Fair Value — We have categorized our financial instruments, based on the degree of subjectivity inherent in the method by which they are valued, into a fair value hierarchy of three levels, as follows: • Level 1 : Inputs are unadjusted, quoted prices in active markets for identical instruments at the measurement date (e.g., U.S. government obligations, which are continually priced on a daily basis, active exchange-traded equity securities, and certain short-term securities). • Level 2 : Inputs (other than quoted prices included within Level 1) that are observable for the instrument either directly or indirectly (e.g., certain corporate and municipal bonds and certain preferred stocks). This includes: (i) quoted prices for similar instruments in active markets, (ii) quoted prices for identical or similar instruments in markets that are not active, (iii) inputs other than quoted prices that are observable for the instruments, and (iv) inputs that are derived principally from or corroborated by observable market data by correlation or other means. • Level 3 : Inputs that are unobservable. Unobservable inputs reflect our subjective evaluation about the assumptions market participants would use in pricing the financial instrument (e.g., certain structured securities and privately held investments). Determining the fair value of the investment portfolio is the responsibility of management. As part of the responsibility, we evaluate whether a market is distressed or inactive in determining the fair value for our portfolio. We review certain market level inputs to evaluate whether sufficient activity, volume, and new issuances exist to create an active market. Based on this evaluation, we concluded that there was sufficient activity related to the sectors and securities for which we obtained valuations. The composition of the investment portfolio by major security type and our outstanding debt was: Fair Value (millions) Level 1 Level 2 Level 3 Total Cost September 30, 2021 Fixed maturities: U.S. government obligations $ 21,142.1 $ 0 $ 0 $ 21,142.1 $ 21,093.1 State and local government obligations 0 2,165.9 0 2,165.9 2,123.8 Corporate debt securities 0 10,874.0 0 10,874.0 10,568.7 Subtotal 21,142.1 13,039.9 0 34,182.0 33,785.6 Asset-backed securities: Residential mortgage-backed 0 627.9 0 627.9 624.4 Commercial mortgage-backed 0 5,789.0 0 5,789.0 5,739.4 Other asset-backed 0 4,262.2 0 4,262.2 4,235.6 Subtotal asset-backed securities 0 10,679.1 0 10,679.1 10,599.4 Redeemable preferred stocks: Financials 0 51.4 0 51.4 50.8 Utilities 0 0 0 0 0 Industrials 10.6 122.6 0 133.2 120.2 Subtotal redeemable preferred stocks 10.6 174.0 0 184.6 171.0 Total fixed maturities 21,152.7 23,893.0 0 45,045.7 44,556.0 Short-term investments 1,078.5 10.2 0 1,088.7 1,088.7 Total available-for-sale securities 22,231.2 23,903.2 0 46,134.4 45,644.7 Equity securities: Nonredeemable preferred stocks: Financials 89.8 1,304.6 76.4 1,470.8 1,399.8 Utilities 0 42.1 0 42.1 40.0 Industrials 0 25.5 34.4 59.9 40.1 Subtotal nonredeemable preferred stocks 89.8 1,372.2 110.8 1,572.8 1,479.9 Common equities: Common stocks 4,513.1 0 54.5 4,567.6 1,225.8 Other risk investments 0 0 12.6 12.6 12.6 Subtotal common equities 4,513.1 0 67.1 4,580.2 1,238.4 Total equity securities 4,602.9 1,372.2 177.9 6,153.0 2,718.3 Total portfolio $ 26,834.1 $ 25,275.4 $ 177.9 $ 52,287.4 $ 48,363.0 Debt $ 0 $ 5,881.7 $ 0 $ 5,881.7 $ 4,898.2 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost September 30, 2020 Fixed maturities: U.S. government obligations $ 10,203.8 $ 0 $ 0 $ 10,203.8 $ 9,835.1 State and local government obligations 0 4,529.8 0 4,529.8 4,382.1 Corporate debt securities 0 10,612.0 0 10,612.0 10,038.7 Subtotal 10,203.8 15,141.8 0 25,345.6 24,255.9 Asset-backed securities: Residential mortgage-backed 0 531.0 0 531.0 525.4 Commercial mortgage-backed 0 6,053.6 0 6,053.6 5,964.4 Other asset-backed 0 4,200.3 0 4,200.3 4,155.8 Subtotal asset-backed securities 0 10,784.9 0 10,784.9 10,645.6 Redeemable preferred stocks: Financials 0 49.2 0 49.2 51.2 Utilities 0 11.3 0 11.3 10.0 Industrials 10.1 112.4 0 122.5 120.1 Subtotal redeemable preferred stocks 10.1 172.9 0 183.0 181.3 Total fixed maturities 10,213.9 26,099.6 0 36,313.5 35,082.8 Short-term investments 4,506.2 161.6 0 4,667.8 4,667.8 Total available-for-sale securities 14,720.1 26,261.2 0 40,981.3 39,750.6 Equity securities: Nonredeemable preferred stocks: Financials 112.6 1,096.1 13.1 1,221.8 1,197.7 Utilities 0 40.7 0 40.7 39.9 Industrials 0 20.5 15.2 35.7 40.1 Subtotal nonredeemable preferred stocks 112.6 1,157.3 28.3 1,298.2 1,277.7 Common equities: Common stocks 3,457.1 0 25.0 3,482.1 1,152.3 Other risk investments 0 0 2.7 2.7 2.7 Subtotal common equities 3,457.1 0 27.7 3,484.8 1,155.0 Total equity securities 3,569.7 1,157.3 56.0 4,783.0 2,432.7 Total portfolio $ 18,289.8 $ 27,418.5 $ 56.0 $ 45,764.3 $ 42,183.3 Debt $ 0 $ 6,687.4 $ 0 $ 6,687.4 $ 5,395.4 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost December 31, 2020 Fixed maturities: U.S. government obligations $ 12,740.0 $ 0 $ 0 $ 12,740.0 $ 12,437.9 State and local government obligations 0 3,221.8 0 3,221.8 3,099.4 Corporate debt securities 0 10,185.2 0 10,185.2 9,579.7 Subtotal 12,740.0 13,407.0 0 26,147.0 25,117.0 Asset-backed securities: Residential mortgage-backed 0 509.5 0 509.5 503.3 Commercial mortgage-backed 0 6,175.1 0 6,175.1 6,042.6 Other asset-backed 0 3,784.6 0 3,784.6 3,745.0 Subtotal asset-backed securities 0 10,469.2 0 10,469.2 10,290.9 Redeemable preferred stocks: Financials 0 51.6 0 51.6 51.1 Utilities 0 11.7 0 11.7 10.0 Industrials 10.8 120.6 0 131.4 120.1 Subtotal redeemable preferred stocks 10.8 183.9 0 194.7 181.2 Total fixed maturities 12,750.8 24,060.1 0 36,810.9 35,589.1 Short-term investments 4,704.9 513.6 0 5,218.5 5,218.5 Total available-for-sale securities 17,455.7 24,573.7 0 42,029.4 40,807.6 Equity securities: Nonredeemable preferred stocks: Financials 117.7 1,212.3 10.0 1,340.0 1,253.6 Utilities 0 41.9 0 41.9 40.0 Industrials 0 24.3 16.7 41.0 40.1 Subtotal nonredeemable preferred stocks 117.7 1,278.5 26.7 1,422.9 1,333.7 Common equities: Common stocks 4,049.9 0 25.0 4,074.9 1,209.2 Other risk investments 0 0 3.1 3.1 3.1 Subtotal common equities 4,049.9 0 28.1 4,078.0 1,212.3 Total equity securities 4,167.6 1,278.5 54.8 5,500.9 2,546.0 Total portfolio $ 21,623.3 $ 25,852.2 $ 54.8 $ 47,530.3 $ 43,353.6 Debt $ 0 $ 6,793.5 $ 0 $ 6,793.5 $ 5,396.1 Our portfolio valuations, excluding short-term investments, classified as either Level 1 or Level 2 in the above tables are priced exclusively by external sources, including: pricing vendors, dealers/market makers, and exchange-quoted prices. Our short-term investments classified as Level 1 are highly liquid, actively marketed, and have a very short duration, primarily 90 days or less to redemption. These securities are held at their original cost, adjusted for any accretion of discount, since that value very closely approximates what an active market participant would be willing to pay for such securities. The remainder of our short-term investments are classified as Level 2 and are not priced externally since these securities continually trade at par value. These securities are classified as Level 2 since they are primarily longer-dated securities issued by municipalities that contain either liquidity facilities or mandatory put features within one year. At September 30, 2021, vendor-quoted prices represented 82% of our Level 1 classifications (excluding short-term investments), compared to 75% and 76% at September 30, 2020 and December 31, 2020, respectively. The securities quoted by vendors in Level 1 primarily represent our holdings in U.S. Treasury Notes, which are frequently traded, and the quotes are considered similar to exchange-traded quotes. The balance of our Level 1 pricing comes from quotes obtained directly from trades made on active exchanges. At September 30, 2021, vendor-quoted prices comprised 97% of our Level 2 classifications (excluding short-term investments), while dealer-quoted prices represented the remaining 3%, compared to 99% and 1%, respectively, at both September 30, 2020 and December 31, 2020. In our process for selecting a source (e.g., dealer or pricing service) to provide pricing for securities in our portfolio, we reviewed documentation from the sources that detailed the pricing techniques and methodologies used by these sources and determined if their policies adequately considered market activity, either based on specific transactions for the particular security type or based on modeling of securities with similar credit quality, duration, yield, and structure that were recently transacted. Once a source is chosen, we continue to monitor any changes or modifications to their processes by reviewing their documentation on internal controls for pricing and market reviews. We review quality control measures of our sources as they become available to determine if any significant changes have occurred from period to period that might indicate issues or concerns regarding their evaluation or market coverage. As part of our pricing procedures, we obtain quotes from more than one source to help us fully evaluate the market price of securities. However, our internal pricing policy is to use a consistent source for individual securities in order to maintain the integrity of our valuation process. Quotes obtained from the sources are not considered binding offers to transact. Under our policy, when a review of the valuation received from our selected source appears to be outside of what is considered market level activity (which is defined as trading at spreads or yields significantly different than those of comparable securities or outside the general sector level movement without a reasonable explanation), we may use an alternate source’s price. To the extent we determine that it may be prudent to substitute one source’s price for another, we will contact the initial source to obtain an understanding of the factors that may be contributing to the significant price variance. To allow us to determine if our initial source is providing a price that is outside of a reasonable range, we review our portfolio pricing on at least a weekly basis. When necessary, we challenge prices from our sources when a price provided does not match our expectations based on our evaluation of market trends and activity. Initially, we perform a review of our portfolio by sector to identify securities whose prices appear outside of a reasonable range. We then perform a more detailed review of fair values for securities disclosed as Level 2. We review dealer bids and quotes for these and/or similar securities to determine the market level context for our valuations. We then evaluate inputs relevant for each class of securities disclosed in the preceding hierarchy tables. For our structured debt securities, including commercial, residential, and asset-backed securities, we evaluate available market-related data for these, and similar securities, related to collateral, delinquencies, and defaults for historical trends and reasonably estimable projections, as well as historical prepayment rates and current prepayment assumptions and cash flow estimates. We further stratify each class of our structured debt securities into more finite sectors (e.g., planned amortization class, first pay, second pay, senior, subordinated, etc.) and use duration, credit quality, and coupon to determine if the fair value is appropriate. For our corporate debt and preferred stock (redeemable and nonredeemable) portfolios, as well as the notes issued by The Progressive Corporation (see Note 4 – Debt ), we review securities by duration, coupon, and credit quality, as well as changes in interest rate and credit spread movements within that stratification. The review also includes recent trades, including: volume traded at various levels that establish a market, issuer specific fundamentals, and industry specific economic news as it comes to light. For our municipal securities (e.g., general obligations, revenue, and housing), we stratify the portfolio to evaluate securities by type, coupon, credit quality, and duration to review price changes relative to credit spread and interest rate changes. Additionally, we look to economic data as it relates to geographic location as an indication of price-to-call or maturity predictors. For municipal housing securities, we look to changes in cash flow projections, both historical and reasonably estimable projections, to understand yield changes and their effect on valuation. Lastly, for our short-term securities, we look at acquisition price relative to the coupon or yield. Since our short-term securities are typically 90 days or less to maturity, with the majority listed in Level 2 being 30 days or less to redemption, we believe that acquisition price is the best estimate of fair value. We also review data assumptions as supplied by our sources to determine if that data is relevant to current market conditions. In addition, we independently review each sector for transaction volumes, new issuances, and changes in spreads, as well as the overall movement of interest rates along the yield curve to determine if sufficient activity and liquidity exists to provide a credible source for our market valuations. During each valuation period, we create internal estimations of portfolio valuation (performance returns), based on current market-related activity (i.e., interest rate and credit spread movements and other credit-related factors) within each major sector of our portfolio. We compare our internally generated portfolio results with those generated based on quotes we receive externally and research material valuation differences. We compare our results to index returns for each major sector adjusting for duration and credit quality differences to better understand our portfolio’s results. Additionally, we review on a monthly basis our external sales transactions and compare the actual final market sales prices to previous market valuation prices. This review provides us further validation that our pricing sources are providing market level prices, since we explain significant price changes (i.e., greater than 2%) as known events occur in the marketplace and affect a security’s price at sale. This analysis provides us with additional comfort regarding the source’s process, the quality of its review, and its willingness to improve its analysis based on feedback from clients. We believe this effort helps ensure that we are reporting the most representative fair values for our securities. Except as described below, our Level 3 securities are also priced externally; however, due to several factors (e.g., nature of the securities, level of activity, and lack of similar securities trading to obtain observable market level inputs), these valuations are more subjective in nature. Certain private equity investments included in the Level 3 category are valued using external pricing supplemented by internal review and analysis. After all the valuations are received and our review is complete, if the inputs used by vendors are determined to not contain sufficient observable market information, we will reclassify the affected security valuations to Level 3. At September 30, 2021 and 2020, and December 31, 2020, we did not have any securities in our fixed-maturity portfolio listed as Level 3. At September 30, 2021, we held seven private nonredeemable preferred securities that were priced internally or by a pricing firm and we held three private nonredeemable preferred securities at both September 30, 2020 and December 31, 2020. At September 30, 2021 and 2020, and December 31, 2020, we held one private common stock that is currently priced by a pricing firm and was previously priced internally. During the third quarter 2021, we reclassified a Level 3 nonredeemable preferred stock to a Level 3 common stock at both September 30, 2020 and December 31, 2020, to reflect a security that converted during 2020 and to conform to the current year presentation. At September 30, 2021, we held six Level 3 other risk investments that were priced at cost or using the equity method, compared to two Level 3 other risk investments that were priced at cost at both September 30, 2020 and December 31, 2020. To the extent we receive prices from external sources for the Level 3 securities, we would review those prices for reasonableness using internally developed assumptions and then compare our derived prices to the prices we received. During 2021 and 2020, there were no material assets or liabilities measured at fair value on a nonrecurring basis. Based on our review, all prices received from external sources for 2021 remained unadjusted. Due to the relative size of the Level 3 securities’ fair values compared to the total portfolio’s fair value, any changes in pricing methodology would not have a significant change in valuation that would materially impact net or comprehensive income. The following tables provide a summary of changes in fair value associated with Level 3 assets for the three and nine months ended September 30, 2021 and 2020: Level 3 Fair Value (millions) Fair Value at June 30, 2021 Calls/ Purchases Sales Net Realized Change in Net Fair Value at September 30, 2021 Equity securities: Nonredeemable preferred stocks: Financials $ 76.4 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 76.4 Industrials 34.4 0 0 0 0 0 0 34.4 Common equities: Common stocks 52.5 0 2.0 0 0 0 0 54.5 Other risk investments 11.6 0 1.0 0 0 0 0 12.6 Total Level 3 securities $ 174.9 $ 0 $ 3.0 $ 0 $ 0 $ 0 $ 0 $ 177.9 Level 3 Fair Value (millions) Fair Value at June 30, 2020 Calls/ Purchases Sales Net Realized Change in Net Fair Value at September 30, 2020 Equity securities: Nonredeemable preferred stocks: Financials 1 $ 38.1 $ (25.0) $ 0 $ 0 $ 0 $ 0 $ 0 $ 13.1 Industrials 15.2 0 0 0 0 0 0 15.2 Common equities: Common stocks 1 0 25.0 0 0 0 0 0 25.0 Other risk investments 0.3 0 2.4 0 0 0 0 2.7 Total Level 3 securities $ 53.6 $ 0 $ 2.4 $ 0 $ 0 $ 0 $ 0 $ 56.0 1 We purchased a nonredeemable preferred stock security in second quarter 2020 that was subject to an automatic conversion to common stock during third quarter 2020. Level 3 Fair Value (millions) Fair Value at December 31, 2020 Calls/ Purchases Sales Net Realized (Gain)/Loss on Sales 1 Change in Net Fair Value at September 30, 2021 Equity securities: Nonredeemable preferred stocks: Financials $ 10.0 $ 0 $ 60.2 $ 0 $ 0 $ 6.2 $ 0 $ 76.4 Industrials 16.7 0 0 0 (4.5) 22.2 0 34.4 Common equities: Common stocks 25.0 0 2.0 0 0 27.5 0 54.5 Other risk investments 2 3.1 0 9.5 0 0 0 0 12.6 Total Level 3 securities $ 54.8 $ 0 $ 71.7 $ 0 $ (4.5) $ 55.9 $ 0 $ 177.9 1 Recognized in conjunction with a merger transaction. 2 Purchases include securities assumed in the Protective Insurance acquisition. Level 3 Fair Value (millions) Fair Value at December 31, 2019 Calls/ Purchases Sales Net Realized Change in Net Fair Value at September 30, 2020 Equity securities: Nonredeemable preferred stocks: Financials 1 $ 27.1 $ (25.0) $ 25.0 $ 0 $ 0 $ (14.0) $ 0 $ 13.1 Industrials 16.0 0 0 0 0 (0.8) 0 15.2 Common equities: Common stocks 1 0 25.0 0 0 0 0 0 25.0 Other risk investments 0.3 0 2.4 0 0 0 0 2.7 Total Level 3 securities $ 43.4 $ 0 $ 27.4 $ 0 $ 0 $ (14.8) $ 0 $ 56.0 1 We purchased a nonredeemable preferred stock security in second quarter 2020 that was subject to an automatic conversion to common stock during third quarter 2020. The following tables provide a summary of the quantitative information about Level 3 fair value measurements for our applicable securities at September 30, 2021 and 2020, and December 31, 2020: Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at September 30, 2021 Valuation Technique Unobservable Input Unobservable Input Assumption Equity securities: Nonredeemable preferred stocks: Financials $ 16.2 Internal price Recent transaction price per share 27.5 Financials 1 22.5 Internal price Unadjusted purchase price per share 20.1 Financials 1 12.5 Internal price Unadjusted purchase price per share 11.8 Financials 1 10.2 Internal price Unadjusted purchase price per share 32.1 Financials 1 15.0 Internal price Unadjusted purchase price per share 6.8 Industrials 10.7 Internal price Recent transaction price per share 5.2 Industrials 23.7 Internal price Recent transaction price per share 9.6 Common equities: Common stocks 54.5 Pricing firm Market approach (guideline public company method) price per share 7.7 Subtotal Level 3 securities 165.3 Pricing exemption securities 12.6 Total Level 3 securities $ 177.9 1 These securities were purchased during second quarter 2021. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at September 30, 2020 Valuation Technique Unobservable Input Unobservable Input Assumption Equity securities: Nonredeemable preferred stocks: Financials $ 13.1 Internal price Market approach (guideline public company method) price per share 4.4 Industrials 1 7.1 Internal price Market approach (guideline public company method) price per share 5.6 Industrials 1 8.1 Internal price Market approach (guideline public company method) price per share 4.0 Common equities: Common stocks 2 25.0 Internal price Unadjusted purchase price per share 3.7 Subtotal Level 3 securities 53.3 Pricing exemption securities 2.7 Total Level 3 securities $ 56.0 1 Modified prior year disclosure to present securities individually to conform to the current year presentation. 2 This security converted from nonredeemable preferred stock to common stock as a result of an automatic conversion during third quarter 2020. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at December 31, 2020 Valuation Technique Unobservable Input Unobservable Input Assumption Equity securities: Nonredeemable preferred stocks: Financials 1 $ 10.0 Internal price Unadjusted purchase price per share 16.9 Industrials 6.9 Pricing firm Market approach (guideline public company method) price per share 5.4 Industrials 9.8 Pricing firm Market approach (guideline public company method) price per share 4.8 Common equities: Common stocks 2 25.0 Internal price Unadjusted purchase price per share 3.7 Subtotal Level 3 securities 51.7 Pricing exemption securities 3.1 Total Level 3 securities $ 54.8 1 This security was purchased in second quarter 2020. 2 This security converted from nonredeemable preferred stock to common stock as a result of an automatic conversion during third quarter 2020.

Debt

Debt	9 Months Ended
Debt	Sep. 30, 2021
Debt Disclosure [Abstract]
Debt	Debt — Debt at each of the balance sheet periods consisted of: September 30, 2021 September 30, 2020 December 31, 2020 (millions) Carrying Fair Carrying Fair Carrying Fair 3.75% Senior Notes due 2021 (issued: $500.0, August 2011) $ 0 $ 0 $ 499.7 $ 515.3 $ 499.8 $ 510.9 2.45% Senior Notes due 2027 (issued: $500.0, August 2016) 497.6 530.6 497.2 532.0 497.3 541.1 6 5/8% Senior Notes due 2029 (issued: $300.0, March 1999) 297.1 390.9 296.8 411.4 296.9 409.4 4.00% Senior Notes due 2029 (issued: $550.0, October 2018) 545.8 627.9 545.3 662.5 545.5 660.4 3.20% Senior Notes due 2030 (issued: $500.0, March 2020) 496.4 543.2 496.0 572.4 496.1 575.5 6.25% Senior Notes due 2032 (issued: $400.0, November 2002) 396.1 545.5 395.9 587.2 396.0 582.0 4.35% Senior Notes due 2044 (issued: $350.0, April 2014) 346.8 433.6 346.7 450.5 346.7 459.7 3.70% Senior Notes due 2045 (issued: $400.0, January 2015) 395.6 456.2 395.5 473.5 395.5 481.0 4.125% Senior Notes due 2047 (issued: $850.0, April 2017) 841.9 1,018.5 841.7 1,078.5 841.7 1,113.1 4.20% Senior Notes due 2048 (issued: $600.0, March 2018) 590.2 736.3 590.0 775.3 590.0 806.7 3.95% Senior Notes due 2050 (issued: $500.0, March 2020) 490.7 599.0 490.6 628.8 490.6 653.7 Total $ 4,898.2 $ 5,881.7 $ 5,395.4 $ 6,687.4 $ 5,396.1 $ 6,793.5 There was no short-term debt outstanding at September 30, 2021. Short-term debt outstanding at September 30, 2020 and December 31, 2020 consisted of the $500 million 3.75% Senior Notes that matured in August 2021. The Progressive Corporation has a line of credit with PNC Bank, National Association (PNC), in the maximum principal amount of $250 million, which has the same terms as the line of credit with PNC that expired in April 2021. See the 2020 Annual Report to Shareholders for terms of this line of credit. We had no borrowings under the line of credit during the periods presented.

Income Taxes

Income Taxes	9 Months Ended
Income Taxes	Sep. 30, 2021
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes — Deferred tax assets and liabilities are recorded based on the difference between the financial statement and tax bases of assets and liabilities at the enacted tax rates. We review our deferred tax assets regularly for recoverability. At September 30, 2021 and 2020, and December 31, 2020, we determined that we did not need a valuation allowance on our gross deferred tax assets. Although realization of the deferred tax assets is not assured, management believes that it is more likely than not the deferred tax assets will be realized based on our expectation that we will be able to fully utilize the deductions that are ultimately recognized for tax purposes. For the nine months ended September 30, 2021, there have been no material changes in our reserve for uncertain tax positions. The effective tax rate for the three and nine months ended September 30, 2021, was 14.7% and 20.5%, respectively, compared to 20.4% and 20.7% for the same periods last year. For the three months ended September 30, 2021, the lower effective tax rate was primarily due to our underwriting loss during the quarter, compared to an underwriting profit in the other periods presented.

Loss and Loss Adjustment Expens

Loss and Loss Adjustment Expense Reserves	9 Months Ended
Loss and Loss Adjustment Expense Reserves	Sep. 30, 2021
Loss and Loss Adjustment Expenses Reserves [Abstract]
Loss and Loss Adjustment Expenses Reserves	Loss and Loss Adjustment Expense Reserves — Activity in the loss and loss adjustment expense reserves is summarized as follows: September 30, (millions) 2021 2020 Balance at January 1 $ 20,265.8 $ 18,105.4 Less reinsurance recoverables on unpaid losses 3,798.2 3,212.2 Net balance at January 1 16,467.6 14,893.2 Net loss and loss adjustment expense reserves acquired 1 732.5 0 Total beginning reserves 17,200.1 14,893.2 Incurred related to: Current year 24,610.8 18,073.6 Prior years 156.8 116.1 Total incurred 24,767.6 18,189.7 Paid related to: Current year 13,650.6 10,615.2 Prior years 7,213.3 6,682.8 Total paid 20,863.9 17,298.0 Net balance at September 30 21,103.8 15,784.9 Plus reinsurance recoverables on unpaid losses 4,822.5 3,804.3 Balance at September 30 $ 25,926.3 $ 19,589.2 1 Net reserves acquired in Protective Insurance acquisition. We experienced unfavorable reserve development of $156.8 million and $116.1 million during the first nine months of 2021 and 2020, respectively, which is reflected as “Incurred related to prior years ” in the table above. Year-to-date September 30, 2021 • The unfavorable prior year reserve development included $109 million attributable to accident year 2019 and $113 million to 2018 and prior accident years, partially offset by favorable development attributable to accident year 2020. • Our personal auto products incurred $48 million of unfavorable loss and loss adjustment expense (LAE) reserve development, with about 90% attributable to the Agency business. The unfavorable development was primarily attributable to a higher than anticipated frequency of reopened personal injury protection (PIP) claims, primarily in Florida, and higher than anticipated bodily injury severity, partially offset by less late reported claims than anticipated for accident year 2020 and lower adjusting expenses than anticipated. • Our Commercial Lines business experienced $91 million of unfavorable development, primarily due to increased injury severity and the emergence of large injury claims, at rates higher than originally anticipated, primarily in Texas and Florida. • Our Property business experienced $31 million of unfavorable development, primarily due to higher than anticipated severity and reopened claims activity in Florida. • Our special lines business experienced $12 million of favorable development. The remaining unfavorable development is attributable to our runoff business. Year-to-date September 30, 2020 • Approximately 80% of the unfavorable prior year reserve development was attributable to accident year 2018, with the remainder primarily related to accident year 2017 and prior accident years. During the second and third quarter 2020, we experienced favorable development on accident year 2019, primarily due to higher than anticipated salvage and subrogation recoveries, which almost fully offset the unfavorable development related to accident year 2019 we experienced during the first quarter 2020. • Our personal auto products incurred $47 million of unfavorable LAE reserve development, with the Agency and Direct auto business each contributing about half. The unfavorable LAE development was primarily attributable to revised estimates of our per claim settlement costs taken during the first quarter. We also experienced higher than anticipated frequency of reopened PIP claims, primarily in Florida, and late reported losses occurring toward the end of 2019 but not reported until 2020, which was partially offset by higher than anticipated salvage and subrogation recoveries. • Our Commercial Lines business experienced $104 million of unfavorable development, primarily due to increased injury severity and the emergence of large injury claims at rates higher than originally anticipated. • Our special lines and Property businesses experienced $22 million and $13 million, respectively, of favorable development.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	9 Months Ended
Supplemental Cash Flow Information	Sep. 30, 2021
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	Supplemental Cash Flow Information — Cash and cash equivalents include bank demand deposits and daily overnight reverse repurchase commitments of funds held in bank demand deposit accounts by certain subsidiaries, and are not considered part of the investment portfolio. The amount of reverse repurchase commitments held by these subsidiaries at September 30, 2021 and 2020, and December 31, 2020, were $134.6 million, $77.2 million, and $93.5 million, respectively. At September 30, 2021, $14.4 million of the restricted cash and cash equivalents on our consolidated balance sheets represents collateral held against unpaid premiums in the form of certificates of deposit, with the remainder representing cash restricted to pay flood claims under the National Flood Insurance Program’s “Write Your Own” program, for which certain subsidiaries are administrators. During the nine months ended September 30, 2021 and 2020, non-cash activity included declared but unpaid common share dividends of $58.5 million and $58.6 million, respectively (see Note 9 – Dividends for further discussion), and operating lease liabilities arising from obtaining right-of-use assets of $79.5 million and $48.5 million. We paid the following in the respective periods: Nine Months Ended September 30, (millions) 2021 2020 Income taxes $ 669.5 $ 1,079.2 Interest 186.3 168.4 Operating lease liabilities 67.8 66.6

Segment Information

Segment Information	9 Months Ended
Segment Information	Sep. 30, 2021
Segment Reporting [Abstract]
Segment Information	Segment Information — Our Personal Lines segment writes insurance for personal autos and recreational vehicles (our special lines products). Our Commercial Lines segment writes auto-related liability and physical damage insurance, workers’ compensation coverage primarily for the transportation industry, and business-related general liability and property insurance, predominately for small businesses. Our Property segment writes residential property insurance for homeowners, other property owners, and renters. Our service businesses provide insurance-related services, including processing Commercial Automobile Insurance Procedures/Plans (CAIP) business and serving as an agent for homeowners, general liability, and workers’ compensation insurance, among other products, through programs in our direct Personal Lines and Commercial Lines businesses. All segment revenues are generated from external customers; all intercompany transactions are eliminated in consolidation. Following are the operating results for the respective periods: Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 (millions) Revenues Pretax Revenues Pretax Revenues Pretax Revenues Pretax Personal Lines Agency $ 4,267.9 $ 41.0 $ 4,001.6 $ 596.5 $ 12,586.4 $ 796.0 $ 11,749.3 $ 1,748.7 Direct 4,690.2 (62.4) 4,309.8 517.6 13,755.8 480.6 12,470.1 1,637.8 Total Personal Lines 1 8,958.1 (21.4) 8,311.4 1,114.1 26,342.2 1,276.6 24,219.4 3,386.5 Commercial Lines 1,877.4 197.5 1,214.8 155.9 4,917.0 556.1 3,532.8 448.2 Property 2 526.5 (222.7) 447.3 (52.9) 1,501.3 (376.7) 1,300.6 (192.4) Other indemnity 3 2.8 (0.3) 0 0 6.8 (0.2) 0 0 Total underwriting operations 11,364.8 (46.9) 9,973.5 1,217.1 32,767.3 1,455.8 29,052.8 3,642.3 Fees and other revenues 4 174.9 NA 151.8 NA 516.8 NA 434.8 NA Service businesses 73.8 1.0 58.8 4.4 202.1 12.1 169.4 14.7 Investments 5 245.7 239.1 763.1 758.4 1,723.7 1,705.2 1,585.3 1,570.8 Interest expense NA (54.2) NA (56.4) NA (167.0) NA (160.8) Consolidated total $ 11,859.2 $ 139.0 $ 10,947.2 $ 1,923.5 $ 35,209.9 $ 3,006.1 $ 31,242.3 $ 5,067.0 NA = Not applicable 1 Personal auto insurance accounted for 94% of the total Personal Lines segment net premiums earned during the three and nine months ended September 30, 2021 and 2020; insurance for our special lines products (e.g., motorcycles, ATVs, RVs, watercraft, and snowmobiles) accounted for the balance of the Personal Lines net premiums earned. 2 For the three and nine months ended September 30, 2021, pretax profit (loss) includes $14.2 million and $42.5 million, respectively, of amortization expense associated with acquisition-related intangible assets attributable to our Property segment, and $14.2 million and $42.8 million for the same periods in 2020. See Note 12 – Goodwill and Intangible Assets for further discussion. 3 Primarily includes Protective Insurance’s run-off business operations. 4 Pretax profit (loss) for fees and other revenues is allocated to operating segments. 5 Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit is net of investment expense. Our management uses underwriting margin and combined ratio as primary measures of underwriting profitability. The underwriting margin is the pretax underwriting profit (loss) expressed as a percentage of net premiums earned (i.e., revenues from underwriting operations). Pretax underwriting profit (loss) is calculated as net premiums earned plus fees and other revenues, less: (i) losses and loss adjustment expenses; (ii) policy acquisition costs; (iii) other underwriting expenses; and (iv) policyholder credits. Combined ratio is the complement of the underwriting margin. Following are the underwriting margins and combined ratios for our underwriting operations for the respective periods: Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Under-writing Combined Under-writing Combined Under-writing Combined Under-writing Combined Personal Lines Agency 1.0 % 99.0 14.9 % 85.1 6.3 % 93.7 14.9 % 85.1 Direct (1.3) 101.3 12.0 88.0 3.5 96.5 13.1 86.9 Total Personal Lines (0.2) 100.2 13.4 86.6 4.9 95.1 14.0 86.0 Commercial Lines 10.5 89.5 12.8 87.2 11.3 88.7 12.7 87.3 Property 1 (42.3) 142.3 (11.8) 111.8 (25.1) 125.1 (14.8) 114.8 Total underwriting operations (0.4) 100.4 12.2 87.8 4.4 95.6 12.5 87.5 1 Included in the three and nine months ended September 30, 2021, is 2.7 points and 2.8 points, respectively, of amortization expense associated with intangible assets and 3.2 points and 3.3 points, respectively, for the three and nine months ended September 30, 2020.

Dividends

Dividends	9 Months Ended
Dividends	Sep. 30, 2021
Dividends [Abstract]
Dividends	Dividends — Following is a summary of our common and preferred share dividends that were declared and/or paid during the nine months ended September 30, 2021 and 2020: (millions, except per share amounts) Amount Declared Payable Per Share Accrued Common - Quarterly Dividends: 1 August 2021 October 2021 $ 0.10 $ 58.5 May 2021 July 2021 0.10 58.5 March 2021 April 2021 0.10 58.5 December 2020 January 2021 0.10 58.6 August 2020 October 2020 0.10 58.6 May 2020 July 2020 0.10 58.5 February 2020 April 2020 0.10 58.5 December 2019 January 2020 0.10 58.5 Common - Annual Variable Dividends: 1 December 2020 January 2021 4.50 2,635.9 December 2019 January 2020 2.25 1,316.9 Preferred Dividends: August 2021 September 2021 26.875 13.4 December 2020 March 2021 26.875 13.4 August 2020 September 2020 26.875 13.4 February 2020 March 2020 26.875 13.4 1 The accrual is based on an estimate of common shares outstanding as of the record date. The dividends accrued at September 30, 2020, were reclassified from accounts payable, accrued expenses, and other liabilities into dividends payable on common shares on the consolidated balance sheets, to conform to the current year presentation. See Note 14 – Dividends in our 2020 Annual Report to Shareholders for a discussion of our common and preferred share dividend policies.

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	9 Months Ended
Other Comprehensive Income (Loss)	Sep. 30, 2021
Equity [Abstract]
Other Comprehensive Income (Loss)	Other Comprehensive Income (Loss) — The components of other comprehensive income (loss), including reclassification adjustments by income statement line item, were as follows: Components of Changes in (millions) Pretax total Total tax After tax total Total net unrealized gains (losses) on securities Net unrealized gains (losses) on forecasted transactions Foreign Balance at June 30, 2021 $ 619.6 $ (136.4) $ 483.2 $ 498.8 $ (15.1) $ (0.5) Other comprehensive income (loss) before reclassifications: Investment securities (117.9) 24.8 (93.1) (93.1) 0 0 Foreign currency translation adjustment (0.3) 0.1 (0.2) 0 0 (0.2) Total other comprehensive income (loss) before reclassifications (118.2) 24.9 (93.3) (93.1) 0 (0.2) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities 46.4 (9.7) 36.7 36.7 0 0 Interest expense (0.2) 0.1 (0.1) 0 (0.1) 0 Total reclassification adjustment for amounts realized in net income 46.2 (9.6) 36.6 36.7 (0.1) 0 Total other comprehensive income (loss) (164.4) 34.5 (129.9) (129.8) 0.1 (0.2) Balance at September 30, 2021 $ 455.2 $ (101.9) $ 353.3 $ 369.0 $ (15.0) $ (0.7) Components of Changes in (millions) Pretax total Total tax After tax total Total net unrealized gains (losses) on securities Net unrealized gains (losses) on forecasted transactions Foreign Balance at June 30, 2020 $ 1,237.3 $ (262.3) $ 975.0 $ 991.0 $ (16.0) $ 0 Other comprehensive income (loss) before reclassifications: Investment securities 163.3 (34.4) 128.9 128.9 0 0 Foreign currency translation adjustment 0 0 0 0 0 0 Total other comprehensive income (loss) before reclassifications 163.3 (34.4) 128.9 128.9 0 0 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities 194.3 (40.8) 153.5 153.5 0 0 Interest expense (0.3) 0.1 (0.2) 0 (0.2) 0 Total reclassification adjustment for amounts realized in net income 194.0 (40.7) 153.3 153.5 (0.2) 0 Total other comprehensive income (loss) (30.7) 6.3 (24.4) (24.6) 0.2 0 Balance at September 30, 2020 $ 1,206.6 $ (256.0) $ 950.6 $ 966.4 $ (15.8) $ 0 Components of Changes in (millions) Pretax total Total tax After tax total Total net unrealized gains (losses) on securities Net unrealized gains (losses) on forecasted transactions Foreign Balance at December 31, 2020 $ 1,187.4 $ (255.7) $ 931.7 $ 947.3 $ (15.6) $ 0 Other comprehensive income (loss) before reclassifications: Investment securities (494.6) 103.9 (390.7) (390.7) 0 0 Foreign currency translation adjustment (0.9) 0.2 (0.7) 0 0 (0.7) Total other comprehensive income (loss) before reclassifications (495.5) 104.1 (391.4) (390.7) 0 (0.7) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities 237.5 (49.9) 187.6 187.6 0 0 Interest expense (0.8) 0.2 (0.6) 0 (0.6) 0 Total reclassification adjustment for amounts realized in net income 236.7 (49.7) 187.0 187.6 (0.6) 0 Total other comprehensive income (loss) (732.2) 153.8 (578.4) (578.3) 0.6 (0.7) Balance at September 30, 2021 $ 455.2 $ (101.9) $ 353.3 $ 369.0 $ (15.0) $ (0.7) Components of Changes in (millions) Pretax total Total tax After tax total Total net Net Foreign Balance at December 31, 2019 $ 439.1 $ (94.7) $ 344.4 $ 360.8 $ (16.4) $ 0 Other comprehensive income (loss) before reclassifications: Investment securities 1,447.7 (304.1) 1,143.6 1,143.6 0 0 Foreign currency translation adjustment 0 0 0 0 0 0 Total other comprehensive income (loss) before reclassifications 1,447.7 (304.1) 1,143.6 1,143.6 0 0 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities 681.0 (143.0) 538.0 538.0 0 0 Interest expense (0.8) 0.2 (0.6) 0 (0.6) 0 Total reclassification adjustment for amounts realized in net income 680.2 (142.8) 537.4 538.0 (0.6) 0 Total other comprehensive income (loss) 767.5 (161.3) 606.2 605.6 0.6 0 Balance at September 30, 2020 $ 1,206.6 $ (256.0) $ 950.6 $ 966.4 $ (15.8) $ 0 In an effort to manage interest rate risk, we may enter into forecasted transactions on Progressive’s debt issuances. We expect to reclassify $0.5 million (pretax) into interest expense during the next 12 months, related to net unrealized losses on forecasted transactions (see Note 4 – Debt in our 2020 Annual Report to Shareholders for further discussion).

Litigation

Litigation	9 Months Ended
Litigation	Sep. 30, 2021
Commitments and Contingencies Disclosure [Abstract]
Litigation	Litigation — The Progressive Corporation and/or its insurance subsidiaries are named as defendants in various lawsuits arising out of claims made under insurance policies written by our insurance subsidiaries in the ordinary course of business. We consider all legal actions relating to such claims in establishing our loss and loss adjustment expense reserves. labor rates paid to auto body repair shops; our insurance product design; employment matters; and cases challenging other aspects of our claims, marketing, pricing, or sales practices or other business operations. Other insurance companies face many of these same issues. The nature and volume of litigation pending against The Progressive Corporation and/or its insurance subsidiaries is similar to that which was disclosed in Note 12 – Litigation in our 2020 Annual Report to Shareholders. We plan to contest the pending lawsuits vigorously, but may pursue settlement negotiations in some cases, as we deem appropriate. Although outcomes of pending cases are uncertain until final disposition, we establish accruals for these lawsuits when it is probable that a loss has been or will be incurred and we can reasonably estimate potential loss exposure, which may include a range of loss. As to lawsuits for which the loss is considered neither probable or estimable, or is considered probable but not estimable, we do not establish an accrual. Nevertheless, we continue to evaluate this pending litigation to determine if any losses not deemed probable and estimable become so, at which point we would establish an accrual at our best estimate of the loss or range of loss. With respect to our pending lawsuits that are not related to claims under insurance policies, the accruals that we have established, if any, were not material at September 30, 2021 or 2020, and there were no material settlements during 2020 or the first nine months of 2021. For most of these lawsuits, we do not consider any losses to be both probable and estimable, and we are unable to estimate a meaningful range of loss, if any, at this time, due to the factors discussed in Note 12 – Litigation in our 2020 Annual Report to Shareholders. In the event that any one or more of these lawsuits results in a substantial judgment against us, or settlement by us, or if our accruals (if any) prove to be inadequate by a significant amount, the resulting liability could have a material adverse effect on our consolidated financial condition, cash flows, and/or results of operations. For a further discussion on our pending litigation and related reserving policies, see Note 12 – Litigation in our 2020 Annual Report to Shareholders.

Goodwill and Intangible Assets

Goodwill and Intangible Assets	9 Months Ended
Goodwill and Intangible Assets	Sep. 30, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	Goodwill and Intangible Assets Goodwill During the nine months ended September 30, 2021, there were no changes to the carrying amount of goodwill. No accumulated goodwill impairment losses exist. We annually assess goodwill for impairment during the fourth quarter or when circumstances warrant a review. Intangible Assets The following table is a summary of the net carrying amount of other intangible assets: (millions) September 30, 2021 September 30, 2020 December 31, 2020 Intangible assets subject to amortization $ 119.5 $ 173.1 $ 159.0 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 131.9 $ 185.5 $ 171.4 1 Indefinite-lived intangible assets are comprised of state insurance and agent licenses. State insurance licenses were previously subject to amortization under superseded accounting guidance and have $0.6 million of accumulated amortization for all periods presented. Intangible assets subject to amortization consisted of the following: September 30, 2021 September 30, 2020 December 31, 2020 (millions) Gross Accumulated Net Gross Accumulated Net Gross Accumulated Net Category: Policies in force $ 256.2 $ 237.9 $ 18.3 $ 256.2 $ 201.3 $ 54.9 $ 256.2 $ 210.4 $ 45.8 Agency relationships 159.2 73.9 85.3 159.2 62.6 96.6 159.2 65.4 93.8 Software rights 69.1 56.2 12.9 69.1 47.5 21.6 69.1 49.7 19.4 Trade name 3.6 0.6 3.0 0 0 0 0 0 0 Total $ 488.1 $ 368.6 $ 119.5 $ 484.5 $ 311.4 $ 173.1 $ 484.5 $ 325.5 $ 159.0 During the second quarter 2021, we recognized a $3.6 million trade name intangible asset, with an estimated two-year life, in connection with the Protective Insurance acquisition.

New Accounting Standards

New Accounting Standards	9 Months Ended
New Accounting Standards	Sep. 30, 2021
Accounting Policies [Abstract]
New Accounting Standards	New Accounting Standards — We did not adopt any new accounting standards during the nine months ended September 30, 2021. We assessed the adoption impacts of recently issued accounting standards by the Financial Accounting Standards Board on our consolidated financial statements as well as material updates to previous assessments, if any, from our Annual Report on Form 10-K for the fiscal year ended December 31, 2020. There were no new material accounting standards issued in the nine months ended September 30, 2021, that impacted The Progressive Corporation.

Acquisition

Acquisition	9 Months Ended
Acquisition	Sep. 30, 2021
Business Combination and Asset Acquisition [Abstract]
Acquisition	Acquisition — On June 1, 2021, The Progressive Corporation, through an indirect subsidiary, acquired all of the outstanding Class A and Class B common shares of Protective Insurance for $23.30 per common share in cash, or approximately $338 million in aggregate. Through its subsidiaries, Protective Insurance provides liability and workers’ compensation coverage for trucking and public transportation fleets, along with trucking industry independent contractors. As a result of the Protective Insurance acquisition, we will be able to expand our capabilities with the expertise Protective Insurance offers in larger fleet and affinity programs, by providing additional product lines for us to add to our portfolio. Protective Insurance and its subsidiaries represented 1% of companywide net premiums written during the third quarter 2021. All assets and liabilities were recorded at fair value at the date of acquisition. If new information is obtained within 12 months from the date of acquisition about facts and circumstances that existed at the acquisition date, we will adjust the amounts previously recorded. We recorded a $3.6 million intangible asset related to the Protective Insurance trade name, which is being amortized over 2 years; no other intangible assets were identified. For income tax purposes, the historical tax bases of the acquired assets and assumed liabilities carried over and were not recorded at fair value; therefore, no tax-basis goodwill was created. At the date of acquisition, Protective Insurance had total assets of $1.7 billion, including investment securities of $1.1 billion; cash, cash equivalents, and restricted cash and equivalents of $24.3 million; reinsurance recoverables of $452.4 million, and liabilities of $1.4 billion, consisting of unearned premiums of $66.9 million and loss and loss adjustment expense reserves of $1.1 billion. In addition, Protective Insurance had outstanding borrowings of $20.0 million under a revolving credit facility agreement that was repaid subsequent to acquisition. All of Protective Insurance’s contingencies were recognized as of the acquisition date. Acquisition-related costs recognized in 2021 were not material to our results of operations. The pro forma financial information assuming the acquisition had occurred as of the beginning of the calendar year prior to the year of acquisition, as well as the revenues and earnings generated during the year of acquisition, were not material for disclosure purposes. The results of operations and financial condition of Protective Insurance have been included in our consolidated statements of comprehensive income and consolidated balance sheets from the acquisition date. For the three and nine months ended September 30, 2021, our consolidated results included total revenue of $144.3 million and $190.8 million, respectively, and net income of $11.1 million and $13.4 million, from Protective Insurance.

Basis of Presentation (Policies

Basis of Presentation (Policies)	9 Months Ended
Basis of Presentation (Policies)	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Financing Receivable, Allowance for Credit Losses, Policy for Uncollectible Amounts	We perform analyses to evaluate our premiums receivables for expected credit losses. See the 2020 Annual Report to Shareholders for a discussion on our premiums receivable allowance for credit loss policy.

Basis of Presentation (Tables)

Basis of Presentation (Tables)	9 Months Ended
Basis of Presentation (Tables)	Sep. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Premium Receivable, Allowance for Credit Loss	The following table summarizes changes in our allowance for credit loss exposure on our premiums receivables: Three Months Nine Months Periods Ended September 30, 2021 2020 2021 2020 (millions) Allowance for credit loss, beginning of period $ 245.2 $ 437.9 $ 356.2 $ 283.2 Allowance acquired during period 1 0 0 3.5 0 Increase (decrease) in allowance 2 107.2 (3.6) 255.5 362.9 Write-offs 3 (95.1) (121.1) (357.9) (332.9) Allowance for credit loss, end of period $ 257.3 $ 313.2 $ 257.3 $ 313.2 1 Represents the amount of the allowance acquired in the Protective Insurance acquisition. 2 Represents the incremental increase (decrease) in other underwriting expenses. 3 Represents portion of allowance that is reversed when premiums receivables are written off.

Investments (Tables)

Investments (Tables)	9 Months Ended
Investments (Tables)	Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]
Schedule of Debt and Equity Securities Reconciliation	The following tables present the composition of our investment portfolio by major security type, consistent with our classification of how we manage, monitor, and measure the portfolio. Our securities are reported in our consolidated balance sheets at fair value. The changes in fair value for our fixed-maturity securities (other than hybrid securities) are reported as a component of accumulated other comprehensive income, net of deferred income taxes, in our consolidated balance sheets. The net holding period gains (losses) reported below represent the inception-to-date changes in fair value of the securities. The changes in the net holding period gains (losses) between periods for the hybrid securities and equity securities are recorded as a component of net realized gains (losses) on securities in our consolidated statements of comprehensive income. ($ in millions) Cost Gross Gross Net Fair % of September 30, 2021 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 21,093.1 $ 144.9 $ (95.9) $ 0 $ 21,142.1 40.3 % State and local government obligations 2,123.8 48.8 (6.7) 0 2,165.9 4.1 Corporate debt securities 10,568.7 314.6 (10.5) 1.2 10,874.0 20.8 Residential mortgage-backed securities 624.4 3.4 (0.9) 1.0 627.9 1.2 Commercial mortgage-backed securities 5,739.4 70.2 (20.6) 0 5,789.0 11.1 Other asset-backed securities 4,235.6 29.4 (2.8) 0 4,262.2 8.2 Redeemable preferred stocks 171.0 1.3 (0.7) 13.0 184.6 0.4 Total fixed maturities 44,556.0 612.6 (138.1) 15.2 45,045.7 86.1 Short-term investments 1,088.7 0 0 0 1,088.7 2.1 Total available-for-sale securities 45,644.7 612.6 (138.1) 15.2 46,134.4 88.2 Equity securities: Nonredeemable preferred stocks 1,479.9 0 0 92.9 1,572.8 3.0 Common equities 1,238.4 0 0 3,341.8 4,580.2 8.8 Total equity securities 2,718.3 0 0 3,434.7 6,153.0 11.8 Total portfolio 1 $ 48,363.0 $ 612.6 $ (138.1) $ 3,449.9 $ 52,287.4 100.0 % ($ in millions) Cost Gross Gross Net Fair % of September 30, 2020 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 9,835.1 $ 370.3 $ (1.6) $ 0 $ 10,203.8 22.3 % State and local government obligations 4,382.1 148.2 (0.5) 0 4,529.8 9.9 Corporate debt securities 10,038.7 574.7 (3.6) 2.2 10,612.0 23.2 Residential mortgage-backed securities 525.4 7.0 (1.4) 0 531.0 1.2 Commercial mortgage-backed securities 5,964.4 117.1 (27.9) 0 6,053.6 13.2 Other asset-backed securities 4,155.8 46.8 (2.3) 0 4,200.3 9.2 Redeemable preferred stocks 181.3 2.5 (3.2) 2.4 183.0 0.4 Total fixed maturities 35,082.8 1,266.6 (40.5) 4.6 36,313.5 79.4 Short-term investments 4,667.8 0 0 0 4,667.8 10.2 Total available-for-sale securities 39,750.6 1,266.6 (40.5) 4.6 40,981.3 89.6 Equity securities 2 : Nonredeemable preferred stocks 1,277.7 0 0 20.5 1,298.2 2.8 Common equities 1,155.0 0 0 2,329.8 3,484.8 7.6 Total equity securities 2,432.7 0 0 2,350.3 4,783.0 10.4 Total portfolio 1 $ 42,183.3 $ 1,266.6 $ (40.5) $ 2,354.9 $ 45,764.3 100.0 % ($ in millions) Cost Gross Gross Net Fair % of December 31, 2020 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 12,437.9 $ 305.8 $ (3.7) $ 0 $ 12,740.0 26.8 % State and local government obligations 3,099.4 123.1 (0.7) 0 3,221.8 6.8 Corporate debt securities 9,579.7 601.7 (0.1) 3.9 10,185.2 21.4 Residential mortgage-backed securities 503.3 7.1 (0.9) 0 509.5 1.1 Commercial mortgage-backed securities 6,042.6 142.5 (10.0) 0 6,175.1 13.0 Other asset-backed securities 3,745.0 40.1 (0.5) 0 3,784.6 7.9 Redeemable preferred stocks 181.2 3.6 (1.4) 11.3 194.7 0.4 Total fixed maturities 35,589.1 1,223.9 (17.3) 15.2 36,810.9 77.4 Short-term investments 5,218.5 0 0 0 5,218.5 11.0 Total available-for-sale securities 40,807.6 1,223.9 (17.3) 15.2 42,029.4 88.4 Equity securities 2 : Nonredeemable preferred stocks 1,333.7 0 0 89.2 1,422.9 3.0 Common equities 1,212.3 0 0 2,865.7 4,078.0 8.6 Total equity securities 2,546.0 0 0 2,954.9 5,500.9 11.6 Total portfolio 1 $ 43,353.6 $ 1,223.9 $ (17.3) $ 2,970.1 $ 47,530.3 100.0 % 1 Our portfolio reflects the effect of net unsettled security transactions; at September 30, 2021, we had $399.7 million in other liabilities, compared to $469.2 million and $95.5 million at September 30, 2020 and December 31, 2020, respectively. The total fair value of the portfolio at September 30, 2021 and 2020, and December 31, 2020, included $2.9 billion, $2.8 billion, and $6.2 billion, respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company, net of unsettled security transactions. During the first nine months of 2021, we used a portion of these investments to pay our common share dividends, repurchase common shares, and pay off debt. 2 Includes a $25.0 million reclassification from nonredeemable preferred stocks to common equities to reflect the prior year conversion of a security and to conform to the current year classification.
Hybrid Instruments Table	Certain securities in our fixed-maturity portfolio are accounted for as hybrid securities because they contain embedded derivatives that are not deemed to be clearly and closely related to the host investments. These securities are reported at fair value: September 30, (millions) 2021 2020 December 31, 2020 Fixed Maturities: Corporate debt securities $ 363.3 $ 101.0 $ 188.4 Residential mortgage-backed securities 217.0 0 0 Other asset-backed securities 104.5 18.7 34.8 Redeemable preferred stocks 133.2 122.5 131.4 Total hybrid securities $ 818.0 $ 242.2 $ 354.6
Investments Classified by Contractual Maturity Date	The composition of fixed maturities by maturity at September 30, 2021, was: (millions) Cost Fair Value Less than one year $ 6,124.5 $ 6,153.2 One to five years 27,606.8 27,968.5 Five to ten years 10,761.3 10,861.0 Ten years or greater 63.4 63.0 Total $ 44,556.0 $ 45,045.7
Schedule of Unrealized Loss on Investments	The following tables show the composition of gross unrealized losses, for which no credit related impairment has been recognized, by major security type and by the length of time that individual securities have been in a continuous unrealized loss position: Total No. of Sec. Total Gross Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized No. of Sec. Fair Unrealized September 30, 2021 U.S. government obligations 73 $ 13,352.8 $ (95.9) 65 $ 12,152.6 $ (70.1) 8 $ 1,200.2 $ (25.8) State and local government obligations 96 710.3 (6.7) 81 635.9 (4.9) 15 74.4 (1.8) Corporate debt securities 227 1,581.3 (10.5) 225 1,551.5 (10.0) 2 29.8 (0.5) Residential mortgage-backed securities 63 149.3 (0.9) 53 135.1 (0.6) 10 14.2 (0.3) Commercial mortgage-backed securities 70 1,758.8 (20.6) 65 1,543.6 (19.9) 5 215.2 (0.7) Other asset-backed securities 114 1,625.9 (2.8) 109 1,614.2 (2.7) 5 11.7 (0.1) Redeemable preferred stocks 1 11.8 (0.7) 0 0 0 1 11.8 (0.7) Total fixed maturities 644 $ 19,190.2 $ (138.1) 598 $ 17,632.9 $ (108.2) 46 $ 1,557.3 $ (29.9) Total No. of Sec. Total Gross Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized No. of Sec. Fair Unrealized September 30, 2020 U.S. government obligations 13 $ 1,993.8 $ (1.6) 13 $ 1,993.8 $ (1.6) 0 $ 0 $ 0 State and local government obligations 45 320.1 (0.5) 45 320.1 (0.5) 0 0 0 Corporate debt securities 28 537.8 (3.6) 28 537.8 (3.6) 0 0 0 Residential mortgage-backed securities 28 86.7 (1.4) 9 38.7 (1.0) 19 48.0 (0.4) Commercial mortgage-backed securities 77 1,794.8 (27.9) 46 1,170.4 (17.3) 31 624.4 (10.6) Other asset-backed securities 22 191.5 (2.3) 11 146.5 (2.1) 11 45.0 (0.2) Redeemable preferred stocks 1 9.3 (3.2) 0 0 0 1 9.3 (3.2) Total fixed maturities 214 $ 4,934.0 $ (40.5) 152 $ 4,207.3 $ (26.1) 62 $ 726.7 $ (14.4) Total No. of Sec. Total Gross Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized No. of Sec. Fair Unrealized December 31, 2020 U.S. government obligations 9 $ 1,511.0 $ (3.7) 9 $ 1,511.0 $ (3.7) 0 $ 0 $ 0 State and local government obligations 30 208.7 (0.7) 30 208.7 (0.7) 0 0 0 Corporate debt securities 7 129.4 (0.1) 7 129.4 (0.1) 0 0 0 Residential mortgage-backed securities 21 44.4 (0.9) 0 0 0 21 44.4 (0.9) Commercial mortgage-backed securities 43 893.3 (10.0) 9 93.6 (0.3) 34 799.7 (9.7) Other asset-backed securities 22 183.7 (0.5) 9 74.4 (0.1) 13 109.3 (0.4) Redeemable preferred stocks 1 11.0 (1.4) 0 0 0 1 11.0 (1.4) Total fixed maturities 133 $ 2,981.5 $ (17.3) 64 $ 2,017.1 $ (4.9) 69 $ 964.4 $ (12.4)
Realized Gain (Loss) on Investments	The components of net realized gains (losses) for the three and nine months ended September 30, were: Three Months Nine Months (millions) 2021 2020 2021 2020 Gross realized gains on security sales Available-for-sale securities: U.S. government obligations $ 19.6 $ 90.2 $ 97.5 $ 565.8 State and local government obligations 6.0 43.9 50.3 58.9 Corporate and other debt securities 23.6 58.8 84.4 125.5 Residential mortgage-backed securities 0 0 0.3 0 Commercial mortgage-backed securities 1.4 5.0 40.7 15.4 Other asset-backed securities 0.3 0.2 1.0 0.2 Redeemable preferred stocks 0 0 1.5 0 Total available-for-sale securities 50.9 198.1 275.7 765.8 Equity securities: Nonredeemable preferred stocks 2.3 0.3 26.0 20.0 Common equities 6.2 2.0 352.3 77.1 Total equity securities 8.5 2.3 378.3 97.1 Subtotal gross realized gains on security sales 59.4 200.4 654.0 862.9 Gross realized losses on security sales Available-for-sale securities: U.S. government obligations (2.0) (1.5) (28.5) (4.8) State and local government obligations 0 0 (3.0) 0 Corporate and other debt securities (0.7) 0 (6.1) (6.5) Residential mortgage-backed securities (0.3) 0 (0.3) 0 Commercial mortgage-backed securities (0.5) (2.9) (1.6) (12.7) Other asset-backed securities 0 0 (0.4) 0 Total available-for-sale securities (3.5) (4.4) (39.9) (24.0) Equity securities: Nonredeemable preferred stocks (0.3) (0.1) (0.7) (7.5) Common equities (1.7) 0 (5.6) (60.3) Total equity securities (2.0) (0.1) (6.3) (67.8) Subtotal gross realized losses on security sales (5.5) (4.5) (46.2) (91.8) Net realized gains (losses) on security sales Available-for-sale securities: U.S. government obligations 17.6 88.7 69.0 561.0 State and local government obligations 6.0 43.9 47.3 58.9 Corporate and other debt securities 22.9 58.8 78.3 119.0 Residential mortgage-backed securities (0.3) 0 0 0 Commercial mortgage-backed securities 0.9 2.1 39.1 2.7 Other asset-backed securities 0.3 0.2 0.6 0.2 Redeemable preferred stocks 0 0 1.5 0 Total available-for-sale securities 47.4 193.7 235.8 741.8 Equity securities: Nonredeemable preferred stocks 2.0 0.2 25.3 12.5 Common equities 4.5 2.0 346.7 16.8 Total equity securities 6.5 2.2 372.0 29.3 Subtotal net realized gains (losses) on security sales 53.9 195.9 607.8 771.1 Net holding period gains (losses) Hybrid securities 1.0 3.2 0 (3.2) Equity securities (16.9) 333.5 479.8 101.9 Subtotal net holding period gains (losses) (15.9) 336.7 479.8 98.7 Other asset impairment (1.2) 0 (3.7) 0 Total net realized gains (losses) on securities $ 36.8 $ 532.6 $ 1,083.9 $ 869.8
Components of Equity Realized Gains (Losses)	The following table reflects our holding period realized gains (losses) on equity securities recognized for the three and nine months ended September 30, 2021 and 2020, for equity securities held at the respective quarter end: Three Months Nine Months (millions) 2021 2020 2021 2020 Total net gains (losses) recognized during the period on equity securities $ (10.4) $ 335.7 $ 851.8 $ 131.2 Less: Net gains (losses) recognized on equity securities sold during the period 6.5 2.2 372.0 29.3 Net holding period gains (losses) recognized during the period on equity securities held at period end $ (16.9) $ 333.5 $ 479.8 $ 101.9
Components of Net Investment Income	The components of net investment income for the three and nine months ended September 30, were: Three Months Nine Months (millions) 2021 2020 2021 2020 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 39.0 $ 36.7 $ 106.8 $ 135.0 State and local government obligations 9.7 18.5 34.6 44.2 Corporate debt securities 70.2 76.6 228.6 211.2 Residential mortgage-backed securities 3.2 0.8 9.4 8.8 Commercial mortgage-backed securities 35.0 37.3 105.2 114.2 Other asset-backed securities 16.1 22.6 47.1 76.6 Redeemable preferred stocks 2.3 2.5 7.1 12.5 Total fixed maturities 175.5 195.0 538.8 602.5 Short-term investments 0.4 6.2 2.6 26.9 Total available-for-sale securities 175.9 201.2 541.4 629.4 Equity securities: Nonredeemable preferred stocks 17.3 15.8 52.5 44.3 Common equities 15.7 13.5 45.9 41.8 Total equity securities 33.0 29.3 98.4 86.1 Investment income 208.9 230.5 639.8 715.5 Investment expenses (6.6) (4.7) (18.5) (14.5) Net investment income $ 202.3 $ 225.8 $ 621.3 $ 701.0

Fair Value (Tables)

Fair Value (Tables)	9 Months Ended
Fair Value (Tables)	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Composition of Investment Portfolio by Major Security Type	The composition of the investment portfolio by major security type and our outstanding debt was: Fair Value (millions) Level 1 Level 2 Level 3 Total Cost September 30, 2021 Fixed maturities: U.S. government obligations $ 21,142.1 $ 0 $ 0 $ 21,142.1 $ 21,093.1 State and local government obligations 0 2,165.9 0 2,165.9 2,123.8 Corporate debt securities 0 10,874.0 0 10,874.0 10,568.7 Subtotal 21,142.1 13,039.9 0 34,182.0 33,785.6 Asset-backed securities: Residential mortgage-backed 0 627.9 0 627.9 624.4 Commercial mortgage-backed 0 5,789.0 0 5,789.0 5,739.4 Other asset-backed 0 4,262.2 0 4,262.2 4,235.6 Subtotal asset-backed securities 0 10,679.1 0 10,679.1 10,599.4 Redeemable preferred stocks: Financials 0 51.4 0 51.4 50.8 Utilities 0 0 0 0 0 Industrials 10.6 122.6 0 133.2 120.2 Subtotal redeemable preferred stocks 10.6 174.0 0 184.6 171.0 Total fixed maturities 21,152.7 23,893.0 0 45,045.7 44,556.0 Short-term investments 1,078.5 10.2 0 1,088.7 1,088.7 Total available-for-sale securities 22,231.2 23,903.2 0 46,134.4 45,644.7 Equity securities: Nonredeemable preferred stocks: Financials 89.8 1,304.6 76.4 1,470.8 1,399.8 Utilities 0 42.1 0 42.1 40.0 Industrials 0 25.5 34.4 59.9 40.1 Subtotal nonredeemable preferred stocks 89.8 1,372.2 110.8 1,572.8 1,479.9 Common equities: Common stocks 4,513.1 0 54.5 4,567.6 1,225.8 Other risk investments 0 0 12.6 12.6 12.6 Subtotal common equities 4,513.1 0 67.1 4,580.2 1,238.4 Total equity securities 4,602.9 1,372.2 177.9 6,153.0 2,718.3 Total portfolio $ 26,834.1 $ 25,275.4 $ 177.9 $ 52,287.4 $ 48,363.0 Debt $ 0 $ 5,881.7 $ 0 $ 5,881.7 $ 4,898.2 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost September 30, 2020 Fixed maturities: U.S. government obligations $ 10,203.8 $ 0 $ 0 $ 10,203.8 $ 9,835.1 State and local government obligations 0 4,529.8 0 4,529.8 4,382.1 Corporate debt securities 0 10,612.0 0 10,612.0 10,038.7 Subtotal 10,203.8 15,141.8 0 25,345.6 24,255.9 Asset-backed securities: Residential mortgage-backed 0 531.0 0 531.0 525.4 Commercial mortgage-backed 0 6,053.6 0 6,053.6 5,964.4 Other asset-backed 0 4,200.3 0 4,200.3 4,155.8 Subtotal asset-backed securities 0 10,784.9 0 10,784.9 10,645.6 Redeemable preferred stocks: Financials 0 49.2 0 49.2 51.2 Utilities 0 11.3 0 11.3 10.0 Industrials 10.1 112.4 0 122.5 120.1 Subtotal redeemable preferred stocks 10.1 172.9 0 183.0 181.3 Total fixed maturities 10,213.9 26,099.6 0 36,313.5 35,082.8 Short-term investments 4,506.2 161.6 0 4,667.8 4,667.8 Total available-for-sale securities 14,720.1 26,261.2 0 40,981.3 39,750.6 Equity securities: Nonredeemable preferred stocks: Financials 112.6 1,096.1 13.1 1,221.8 1,197.7 Utilities 0 40.7 0 40.7 39.9 Industrials 0 20.5 15.2 35.7 40.1 Subtotal nonredeemable preferred stocks 112.6 1,157.3 28.3 1,298.2 1,277.7 Common equities: Common stocks 3,457.1 0 25.0 3,482.1 1,152.3 Other risk investments 0 0 2.7 2.7 2.7 Subtotal common equities 3,457.1 0 27.7 3,484.8 1,155.0 Total equity securities 3,569.7 1,157.3 56.0 4,783.0 2,432.7 Total portfolio $ 18,289.8 $ 27,418.5 $ 56.0 $ 45,764.3 $ 42,183.3 Debt $ 0 $ 6,687.4 $ 0 $ 6,687.4 $ 5,395.4 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost December 31, 2020 Fixed maturities: U.S. government obligations $ 12,740.0 $ 0 $ 0 $ 12,740.0 $ 12,437.9 State and local government obligations 0 3,221.8 0 3,221.8 3,099.4 Corporate debt securities 0 10,185.2 0 10,185.2 9,579.7 Subtotal 12,740.0 13,407.0 0 26,147.0 25,117.0 Asset-backed securities: Residential mortgage-backed 0 509.5 0 509.5 503.3 Commercial mortgage-backed 0 6,175.1 0 6,175.1 6,042.6 Other asset-backed 0 3,784.6 0 3,784.6 3,745.0 Subtotal asset-backed securities 0 10,469.2 0 10,469.2 10,290.9 Redeemable preferred stocks: Financials 0 51.6 0 51.6 51.1 Utilities 0 11.7 0 11.7 10.0 Industrials 10.8 120.6 0 131.4 120.1 Subtotal redeemable preferred stocks 10.8 183.9 0 194.7 181.2 Total fixed maturities 12,750.8 24,060.1 0 36,810.9 35,589.1 Short-term investments 4,704.9 513.6 0 5,218.5 5,218.5 Total available-for-sale securities 17,455.7 24,573.7 0 42,029.4 40,807.6 Equity securities: Nonredeemable preferred stocks: Financials 117.7 1,212.3 10.0 1,340.0 1,253.6 Utilities 0 41.9 0 41.9 40.0 Industrials 0 24.3 16.7 41.0 40.1 Subtotal nonredeemable preferred stocks 117.7 1,278.5 26.7 1,422.9 1,333.7 Common equities: Common stocks 4,049.9 0 25.0 4,074.9 1,209.2 Other risk investments 0 0 3.1 3.1 3.1 Subtotal common equities 4,049.9 0 28.1 4,078.0 1,212.3 Total equity securities 4,167.6 1,278.5 54.8 5,500.9 2,546.0 Total portfolio $ 21,623.3 $ 25,852.2 $ 54.8 $ 47,530.3 $ 43,353.6 Debt $ 0 $ 6,793.5 $ 0 $ 6,793.5 $ 5,396.1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation	The following tables provide a summary of changes in fair value associated with Level 3 assets for the three and nine months ended September 30, 2021 and 2020: Level 3 Fair Value (millions) Fair Value at June 30, 2021 Calls/ Purchases Sales Net Realized Change in Net Fair Value at September 30, 2021 Equity securities: Nonredeemable preferred stocks: Financials $ 76.4 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 76.4 Industrials 34.4 0 0 0 0 0 0 34.4 Common equities: Common stocks 52.5 0 2.0 0 0 0 0 54.5 Other risk investments 11.6 0 1.0 0 0 0 0 12.6 Total Level 3 securities $ 174.9 $ 0 $ 3.0 $ 0 $ 0 $ 0 $ 0 $ 177.9 Level 3 Fair Value (millions) Fair Value at June 30, 2020 Calls/ Purchases Sales Net Realized Change in Net Fair Value at September 30, 2020 Equity securities: Nonredeemable preferred stocks: Financials 1 $ 38.1 $ (25.0) $ 0 $ 0 $ 0 $ 0 $ 0 $ 13.1 Industrials 15.2 0 0 0 0 0 0 15.2 Common equities: Common stocks 1 0 25.0 0 0 0 0 0 25.0 Other risk investments 0.3 0 2.4 0 0 0 0 2.7 Total Level 3 securities $ 53.6 $ 0 $ 2.4 $ 0 $ 0 $ 0 $ 0 $ 56.0 1 We purchased a nonredeemable preferred stock security in second quarter 2020 that was subject to an automatic conversion to common stock during third quarter 2020. Level 3 Fair Value (millions) Fair Value at December 31, 2020 Calls/ Purchases Sales Net Realized (Gain)/Loss on Sales 1 Change in Net Fair Value at September 30, 2021 Equity securities: Nonredeemable preferred stocks: Financials $ 10.0 $ 0 $ 60.2 $ 0 $ 0 $ 6.2 $ 0 $ 76.4 Industrials 16.7 0 0 0 (4.5) 22.2 0 34.4 Common equities: Common stocks 25.0 0 2.0 0 0 27.5 0 54.5 Other risk investments 2 3.1 0 9.5 0 0 0 0 12.6 Total Level 3 securities $ 54.8 $ 0 $ 71.7 $ 0 $ (4.5) $ 55.9 $ 0 $ 177.9 1 Recognized in conjunction with a merger transaction. 2 Purchases include securities assumed in the Protective Insurance acquisition. Level 3 Fair Value (millions) Fair Value at December 31, 2019 Calls/ Purchases Sales Net Realized Change in Net Fair Value at September 30, 2020 Equity securities: Nonredeemable preferred stocks: Financials 1 $ 27.1 $ (25.0) $ 25.0 $ 0 $ 0 $ (14.0) $ 0 $ 13.1 Industrials 16.0 0 0 0 0 (0.8) 0 15.2 Common equities: Common stocks 1 0 25.0 0 0 0 0 0 25.0 Other risk investments 0.3 0 2.4 0 0 0 0 2.7 Total Level 3 securities $ 43.4 $ 0 $ 27.4 $ 0 $ 0 $ (14.8) $ 0 $ 56.0
Fair Value Measurement Inputs and Valuation Techniques	The following tables provide a summary of the quantitative information about Level 3 fair value measurements for our applicable securities at September 30, 2021 and 2020, and December 31, 2020: Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at September 30, 2021 Valuation Technique Unobservable Input Unobservable Input Assumption Equity securities: Nonredeemable preferred stocks: Financials $ 16.2 Internal price Recent transaction price per share 27.5 Financials 1 22.5 Internal price Unadjusted purchase price per share 20.1 Financials 1 12.5 Internal price Unadjusted purchase price per share 11.8 Financials 1 10.2 Internal price Unadjusted purchase price per share 32.1 Financials 1 15.0 Internal price Unadjusted purchase price per share 6.8 Industrials 10.7 Internal price Recent transaction price per share 5.2 Industrials 23.7 Internal price Recent transaction price per share 9.6 Common equities: Common stocks 54.5 Pricing firm Market approach (guideline public company method) price per share 7.7 Subtotal Level 3 securities 165.3 Pricing exemption securities 12.6 Total Level 3 securities $ 177.9 1 These securities were purchased during second quarter 2021. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at September 30, 2020 Valuation Technique Unobservable Input Unobservable Input Assumption Equity securities: Nonredeemable preferred stocks: Financials $ 13.1 Internal price Market approach (guideline public company method) price per share 4.4 Industrials 1 7.1 Internal price Market approach (guideline public company method) price per share 5.6 Industrials 1 8.1 Internal price Market approach (guideline public company method) price per share 4.0 Common equities: Common stocks 2 25.0 Internal price Unadjusted purchase price per share 3.7 Subtotal Level 3 securities 53.3 Pricing exemption securities 2.7 Total Level 3 securities $ 56.0 1 Modified prior year disclosure to present securities individually to conform to the current year presentation. 2 This security converted from nonredeemable preferred stock to common stock as a result of an automatic conversion during third quarter 2020. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at December 31, 2020 Valuation Technique Unobservable Input Unobservable Input Assumption Equity securities: Nonredeemable preferred stocks: Financials 1 $ 10.0 Internal price Unadjusted purchase price per share 16.9 Industrials 6.9 Pricing firm Market approach (guideline public company method) price per share 5.4 Industrials 9.8 Pricing firm Market approach (guideline public company method) price per share 4.8 Common equities: Common stocks 2 25.0 Internal price Unadjusted purchase price per share 3.7 Subtotal Level 3 securities 51.7 Pricing exemption securities 3.1 Total Level 3 securities $ 54.8 1 This security was purchased in second quarter 2020. 2 This security converted from nonredeemable preferred stock to common stock as a result of an automatic conversion during third quarter 2020.

Debt (Tables)

Debt (Tables)	9 Months Ended
Debt (Tables)	Sep. 30, 2021
Debt Disclosure [Abstract]
Debt Table	Debt at each of the balance sheet periods consisted of: September 30, 2021 September 30, 2020 December 31, 2020 (millions) Carrying Fair Carrying Fair Carrying Fair 3.75% Senior Notes due 2021 (issued: $500.0, August 2011) $ 0 $ 0 $ 499.7 $ 515.3 $ 499.8 $ 510.9 2.45% Senior Notes due 2027 (issued: $500.0, August 2016) 497.6 530.6 497.2 532.0 497.3 541.1 6 5/8% Senior Notes due 2029 (issued: $300.0, March 1999) 297.1 390.9 296.8 411.4 296.9 409.4 4.00% Senior Notes due 2029 (issued: $550.0, October 2018) 545.8 627.9 545.3 662.5 545.5 660.4 3.20% Senior Notes due 2030 (issued: $500.0, March 2020) 496.4 543.2 496.0 572.4 496.1 575.5 6.25% Senior Notes due 2032 (issued: $400.0, November 2002) 396.1 545.5 395.9 587.2 396.0 582.0 4.35% Senior Notes due 2044 (issued: $350.0, April 2014) 346.8 433.6 346.7 450.5 346.7 459.7 3.70% Senior Notes due 2045 (issued: $400.0, January 2015) 395.6 456.2 395.5 473.5 395.5 481.0 4.125% Senior Notes due 2047 (issued: $850.0, April 2017) 841.9 1,018.5 841.7 1,078.5 841.7 1,113.1 4.20% Senior Notes due 2048 (issued: $600.0, March 2018) 590.2 736.3 590.0 775.3 590.0 806.7 3.95% Senior Notes due 2050 (issued: $500.0, March 2020) 490.7 599.0 490.6 628.8 490.6 653.7 Total $ 4,898.2 $ 5,881.7 $ 5,395.4 $ 6,687.4 $ 5,396.1 $ 6,793.5

Loss and Loss Adjustment Expe_2

Loss and Loss Adjustment Expense Reserves (Tables)	9 Months Ended
Loss and Loss Adjustment Expense Reserves (Tables)	Sep. 30, 2021
Loss and Loss Adjustment Expenses Reserves [Abstract]
Schedule of Liability for Unpaid Claims and Claims Adjustment Expense	Activity in the loss and loss adjustment expense reserves is summarized as follows: September 30, (millions) 2021 2020 Balance at January 1 $ 20,265.8 $ 18,105.4 Less reinsurance recoverables on unpaid losses 3,798.2 3,212.2 Net balance at January 1 16,467.6 14,893.2 Net loss and loss adjustment expense reserves acquired 1 732.5 0 Total beginning reserves 17,200.1 14,893.2 Incurred related to: Current year 24,610.8 18,073.6 Prior years 156.8 116.1 Total incurred 24,767.6 18,189.7 Paid related to: Current year 13,650.6 10,615.2 Prior years 7,213.3 6,682.8 Total paid 20,863.9 17,298.0 Net balance at September 30 21,103.8 15,784.9 Plus reinsurance recoverables on unpaid losses 4,822.5 3,804.3 Balance at September 30 $ 25,926.3 $ 19,589.2 1 Net reserves acquired in Protective Insurance acquisition.

Supplemental Cash Flow Inform_2

Supplemental Cash Flow Information (Tables)	9 Months Ended
Supplemental Cash Flow Information (Tables)	Sep. 30, 2021
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information Table	We paid the following in the respective periods: Nine Months Ended September 30, (millions) 2021 2020 Income taxes $ 669.5 $ 1,079.2 Interest 186.3 168.4 Operating lease liabilities 67.8 66.6

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
Segment Information (Tables)	Sep. 30, 2021
Segment Reporting [Abstract]
Reconciliation of Revenue and operating Income from Segments to Consolidated	Following are the operating results for the respective periods: Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 (millions) Revenues Pretax Revenues Pretax Revenues Pretax Revenues Pretax Personal Lines Agency $ 4,267.9 $ 41.0 $ 4,001.6 $ 596.5 $ 12,586.4 $ 796.0 $ 11,749.3 $ 1,748.7 Direct 4,690.2 (62.4) 4,309.8 517.6 13,755.8 480.6 12,470.1 1,637.8 Total Personal Lines 1 8,958.1 (21.4) 8,311.4 1,114.1 26,342.2 1,276.6 24,219.4 3,386.5 Commercial Lines 1,877.4 197.5 1,214.8 155.9 4,917.0 556.1 3,532.8 448.2 Property 2 526.5 (222.7) 447.3 (52.9) 1,501.3 (376.7) 1,300.6 (192.4) Other indemnity 3 2.8 (0.3) 0 0 6.8 (0.2) 0 0 Total underwriting operations 11,364.8 (46.9) 9,973.5 1,217.1 32,767.3 1,455.8 29,052.8 3,642.3 Fees and other revenues 4 174.9 NA 151.8 NA 516.8 NA 434.8 NA Service businesses 73.8 1.0 58.8 4.4 202.1 12.1 169.4 14.7 Investments 5 245.7 239.1 763.1 758.4 1,723.7 1,705.2 1,585.3 1,570.8 Interest expense NA (54.2) NA (56.4) NA (167.0) NA (160.8) Consolidated total $ 11,859.2 $ 139.0 $ 10,947.2 $ 1,923.5 $ 35,209.9 $ 3,006.1 $ 31,242.3 $ 5,067.0 NA = Not applicable 1 Personal auto insurance accounted for 94% of the total Personal Lines segment net premiums earned during the three and nine months ended September 30, 2021 and 2020; insurance for our special lines products (e.g., motorcycles, ATVs, RVs, watercraft, and snowmobiles) accounted for the balance of the Personal Lines net premiums earned. 2 For the three and nine months ended September 30, 2021, pretax profit (loss) includes $14.2 million and $42.5 million, respectively, of amortization expense associated with acquisition-related intangible assets attributable to our Property segment, and $14.2 million and $42.8 million for the same periods in 2020. See Note 12 – Goodwill and Intangible Assets for further discussion. 3 Primarily includes Protective Insurance’s run-off business operations. 4 Pretax profit (loss) for fees and other revenues is allocated to operating segments. 5 Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit is net of investment expense.
Underwriting Margins and Combined Ratios for our Underwriting Operations	Following are the underwriting margins and combined ratios for our underwriting operations for the respective periods: Three Months Ended September 30, Nine Months Ended September 30, 2021 2020 2021 2020 Under-writing Combined Under-writing Combined Under-writing Combined Under-writing Combined Personal Lines Agency 1.0 % 99.0 14.9 % 85.1 6.3 % 93.7 14.9 % 85.1 Direct (1.3) 101.3 12.0 88.0 3.5 96.5 13.1 86.9 Total Personal Lines (0.2) 100.2 13.4 86.6 4.9 95.1 14.0 86.0 Commercial Lines 10.5 89.5 12.8 87.2 11.3 88.7 12.7 87.3 Property 1 (42.3) 142.3 (11.8) 111.8 (25.1) 125.1 (14.8) 114.8 Total underwriting operations (0.4) 100.4 12.2 87.8 4.4 95.6 12.5 87.5 1 Included in the three and nine months ended September 30, 2021, is 2.7 points and 2.8 points, respectively, of amortization expense associated with intangible assets and 3.2 points and 3.3 points, respectively, for the three and nine months ended September 30, 2020.

Dividends (Tables)

Dividends (Tables)	9 Months Ended
Dividends (Tables)	Sep. 30, 2021
Dividends [Abstract]
Dividends	Following is a summary of our common and preferred share dividends that were declared and/or paid during the nine months ended September 30, 2021 and 2020: (millions, except per share amounts) Amount Declared Payable Per Share Accrued Common - Quarterly Dividends: 1 August 2021 October 2021 $ 0.10 $ 58.5 May 2021 July 2021 0.10 58.5 March 2021 April 2021 0.10 58.5 December 2020 January 2021 0.10 58.6 August 2020 October 2020 0.10 58.6 May 2020 July 2020 0.10 58.5 February 2020 April 2020 0.10 58.5 December 2019 January 2020 0.10 58.5 Common - Annual Variable Dividends: 1 December 2020 January 2021 4.50 2,635.9 December 2019 January 2020 2.25 1,316.9 Preferred Dividends: August 2021 September 2021 26.875 13.4 December 2020 March 2021 26.875 13.4 August 2020 September 2020 26.875 13.4 February 2020 March 2020 26.875 13.4 1 The accrual is based on an estimate of common shares outstanding as of the record date. The dividends accrued at September 30, 2020, were reclassified from accounts payable, accrued expenses, and other liabilities into dividends payable on common shares on the consolidated balance sheets, to conform to the current year presentation.

Other Comprehensive Income (L_2

Other Comprehensive Income (Loss) (Tables)	9 Months Ended
Other Comprehensive Income (Loss) (Tables)	Sep. 30, 2021
Equity [Abstract]
Components of Other Comprehensive Income (Loss)	The components of other comprehensive income (loss), including reclassification adjustments by income statement line item, were as follows: Components of Changes in (millions) Pretax total Total tax After tax total Total net unrealized gains (losses) on securities Net unrealized gains (losses) on forecasted transactions Foreign Balance at June 30, 2021 $ 619.6 $ (136.4) $ 483.2 $ 498.8 $ (15.1) $ (0.5) Other comprehensive income (loss) before reclassifications: Investment securities (117.9) 24.8 (93.1) (93.1) 0 0 Foreign currency translation adjustment (0.3) 0.1 (0.2) 0 0 (0.2) Total other comprehensive income (loss) before reclassifications (118.2) 24.9 (93.3) (93.1) 0 (0.2) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities 46.4 (9.7) 36.7 36.7 0 0 Interest expense (0.2) 0.1 (0.1) 0 (0.1) 0 Total reclassification adjustment for amounts realized in net income 46.2 (9.6) 36.6 36.7 (0.1) 0 Total other comprehensive income (loss) (164.4) 34.5 (129.9) (129.8) 0.1 (0.2) Balance at September 30, 2021 $ 455.2 $ (101.9) $ 353.3 $ 369.0 $ (15.0) $ (0.7) Components of Changes in (millions) Pretax total Total tax After tax total Total net unrealized gains (losses) on securities Net unrealized gains (losses) on forecasted transactions Foreign Balance at June 30, 2020 $ 1,237.3 $ (262.3) $ 975.0 $ 991.0 $ (16.0) $ 0 Other comprehensive income (loss) before reclassifications: Investment securities 163.3 (34.4) 128.9 128.9 0 0 Foreign currency translation adjustment 0 0 0 0 0 0 Total other comprehensive income (loss) before reclassifications 163.3 (34.4) 128.9 128.9 0 0 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities 194.3 (40.8) 153.5 153.5 0 0 Interest expense (0.3) 0.1 (0.2) 0 (0.2) 0 Total reclassification adjustment for amounts realized in net income 194.0 (40.7) 153.3 153.5 (0.2) 0 Total other comprehensive income (loss) (30.7) 6.3 (24.4) (24.6) 0.2 0 Balance at September 30, 2020 $ 1,206.6 $ (256.0) $ 950.6 $ 966.4 $ (15.8) $ 0 Components of Changes in (millions) Pretax total Total tax After tax total Total net unrealized gains (losses) on securities Net unrealized gains (losses) on forecasted transactions Foreign Balance at December 31, 2020 $ 1,187.4 $ (255.7) $ 931.7 $ 947.3 $ (15.6) $ 0 Other comprehensive income (loss) before reclassifications: Investment securities (494.6) 103.9 (390.7) (390.7) 0 0 Foreign currency translation adjustment (0.9) 0.2 (0.7) 0 0 (0.7) Total other comprehensive income (loss) before reclassifications (495.5) 104.1 (391.4) (390.7) 0 (0.7) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities 237.5 (49.9) 187.6 187.6 0 0 Interest expense (0.8) 0.2 (0.6) 0 (0.6) 0 Total reclassification adjustment for amounts realized in net income 236.7 (49.7) 187.0 187.6 (0.6) 0 Total other comprehensive income (loss) (732.2) 153.8 (578.4) (578.3) 0.6 (0.7) Balance at September 30, 2021 $ 455.2 $ (101.9) $ 353.3 $ 369.0 $ (15.0) $ (0.7) Components of Changes in (millions) Pretax total Total tax After tax total Total net Net Foreign Balance at December 31, 2019 $ 439.1 $ (94.7) $ 344.4 $ 360.8 $ (16.4) $ 0 Other comprehensive income (loss) before reclassifications: Investment securities 1,447.7 (304.1) 1,143.6 1,143.6 0 0 Foreign currency translation adjustment 0 0 0 0 0 0 Total other comprehensive income (loss) before reclassifications 1,447.7 (304.1) 1,143.6 1,143.6 0 0 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities 681.0 (143.0) 538.0 538.0 0 0 Interest expense (0.8) 0.2 (0.6) 0 (0.6) 0 Total reclassification adjustment for amounts realized in net income 680.2 (142.8) 537.4 538.0 (0.6) 0 Total other comprehensive income (loss) 767.5 (161.3) 606.2 605.6 0.6 0 Balance at September 30, 2020 $ 1,206.6 $ (256.0) $ 950.6 $ 966.4 $ (15.8) $ 0

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	9 Months Ended
Goodwill and Intangible Assets (Tables)	Sep. 30, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Indefinite-Lived Intangible Assets	The following table is a summary of the net carrying amount of other intangible assets: (millions) September 30, 2021 September 30, 2020 December 31, 2020 Intangible assets subject to amortization $ 119.5 $ 173.1 $ 159.0 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 131.9 $ 185.5 $ 171.4
Intangible Assets Subject to Amortization	The following table is a summary of the net carrying amount of other intangible assets: (millions) September 30, 2021 September 30, 2020 December 31, 2020 Intangible assets subject to amortization $ 119.5 $ 173.1 $ 159.0 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 131.9 $ 185.5 $ 171.4 1 Indefinite-lived intangible assets are comprised of state insurance and agent licenses. State insurance licenses were previously subject to amortization under superseded accounting guidance and have $0.6 million of accumulated amortization for all periods presented. Intangible assets subject to amortization consisted of the following: September 30, 2021 September 30, 2020 December 31, 2020 (millions) Gross Accumulated Net Gross Accumulated Net Gross Accumulated Net Category: Policies in force $ 256.2 $ 237.9 $ 18.3 $ 256.2 $ 201.3 $ 54.9 $ 256.2 $ 210.4 $ 45.8 Agency relationships 159.2 73.9 85.3 159.2 62.6 96.6 159.2 65.4 93.8 Software rights 69.1 56.2 12.9 69.1 47.5 21.6 69.1 49.7 19.4 Trade name 3.6 0.6 3.0 0 0 0 0 0 0 Total $ 488.1 $ 368.6 $ 119.5 $ 484.5 $ 311.4 $ 173.1 $ 484.5 $ 325.5 $ 159.0

Basis of Presentation - Reporti

Basis of Presentation - Reporting and Accounting Policies - Allowance for Credit Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Allowance for Credit Loss [Abstract]
Premium Receivable, Allowance for Credit Loss, Beginning Balance	$ 245.2	$ 437.9	$ 356.2	$ 283.2
Allowance acquired during period	0	0	3.5	0
Increase in allowance	107.2	(3.6)	255.5	362.9
Write-offs	(95.1)	(121.1)	(357.9)	(332.9)
Allowance for credit loss, end of period	$ 257.3	$ 313.2	$ 257.3	$ 313.2

Basis of Presentation Basis of

Basis of Presentation Basis of Presentation-Additional Information (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Assets held-for-sale	$ 12.6	$ 56.6	$ 27.1

Investments Investment Portfoli

Investments Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 45,644.7	$ 40,807.6	$ 39,750.6
Gross Unrealized Gains	612.6	1,223.9	1,266.6
Gross Unrealized Losses	(138.1)	(17.3)	(40.5)
Net Holding Period Gains (Losses)	15.2	15.2	4.6
Total available-for-sale securities	$ 46,134.4	$ 42,029.4	$ 40,981.3
% of Total Fair Value	88.20%	88.40%	89.60%
Equity securities, at Fair Value
Cost	$ 2,718.3	$ 2,546	$ 2,432.7
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	3,434.7	2,954.9	2,350.3
Fair Value	$ 6,153	$ 5,500.9	$ 4,783
% of Total Fair Value	11.80%	11.60%	10.40%
Debt and Equity Securities, at Fair Value
Cost	$ 48,363	$ 43,353.6	$ 42,183.3
Gross Unrealized Gains	612.6	1,223.9	1,266.6
Gross Unrealized Losses	(138.1)	(17.3)	(40.5)
Net Holding Period Gains (Losses)	3,449.9	2,970.1	2,354.9
Fair Value	$ 52,287.4	$ 47,530.3	$ 45,764.3
% of Total Fair Value	100.00%	100.00%	100.00%
Unsettled security transactions, payable	$ 399.7	$ 95.5	$ 469.2
Total fair value of non-insurance subsidiary portfolio	2,900	6,200	2,800
Reclassification, Equity Conversion		25	25
Nonredeemable preferred stocks
Equity securities, at Fair Value
Cost	1,479.9	1,333.7	1,277.7
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	92.9	89.2	20.5
Fair Value	$ 1,572.8	$ 1,422.9	$ 1,298.2
% of Total Fair Value	3.00%	3.00%	2.80%
Common equities
Equity securities, at Fair Value
Cost	$ 1,238.4	$ 1,212.3	$ 1,155
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	3,341.8	2,865.7	2,329.8
Fair Value	$ 4,580.2	$ 4,078	$ 3,484.8
% of Total Fair Value	8.80%	8.60%	7.60%
Total fixed maturities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 44,556	$ 35,589.1	$ 35,082.8
Gross Unrealized Gains	612.6	1,223.9	1,266.6
Gross Unrealized Losses	(138.1)	(17.3)	(40.5)
Net Holding Period Gains (Losses)	15.2	15.2	4.6
Total available-for-sale securities	$ 45,045.7	$ 36,810.9	$ 36,313.5
% of Total Fair Value	86.10%	77.40%	79.40%
Total fixed maturities \| U.S. government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 21,093.1	$ 12,437.9	$ 9,835.1
Gross Unrealized Gains	144.9	305.8	370.3
Gross Unrealized Losses	(95.9)	(3.7)	(1.6)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 21,142.1	$ 12,740	$ 10,203.8
% of Total Fair Value	40.30%	26.80%	22.30%
Total fixed maturities \| State and local government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 2,123.8	$ 3,099.4	$ 4,382.1
Gross Unrealized Gains	48.8	123.1	148.2
Gross Unrealized Losses	(6.7)	(0.7)	(0.5)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 2,165.9	$ 3,221.8	$ 4,529.8
% of Total Fair Value	4.10%	6.80%	9.90%
Total fixed maturities \| Corporate debt securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 10,568.7	$ 9,579.7	$ 10,038.7
Gross Unrealized Gains	314.6	601.7	574.7
Gross Unrealized Losses	(10.5)	(0.1)	(3.6)
Net Holding Period Gains (Losses)	1.2	3.9	2.2
Total available-for-sale securities	$ 10,874	$ 10,185.2	$ 10,612
% of Total Fair Value	20.80%	21.40%	23.20%
Total fixed maturities \| Residential mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 624.4	$ 503.3	$ 525.4
Gross Unrealized Gains	3.4	7.1	7
Gross Unrealized Losses	(0.9)	(0.9)	(1.4)
Net Holding Period Gains (Losses)	1	0	0
Total available-for-sale securities	$ 627.9	$ 509.5	$ 531
% of Total Fair Value	1.20%	1.10%	1.20%
Total fixed maturities \| Commercial mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 5,739.4	$ 6,042.6	$ 5,964.4
Gross Unrealized Gains	70.2	142.5	117.1
Gross Unrealized Losses	(20.6)	(10)	(27.9)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 5,789	$ 6,175.1	$ 6,053.6
% of Total Fair Value	11.10%	13.00%	13.20%
Total fixed maturities \| Other asset-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 4,235.6	$ 3,745	$ 4,155.8
Gross Unrealized Gains	29.4	40.1	46.8
Gross Unrealized Losses	(2.8)	(0.5)	(2.3)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 4,262.2	$ 3,784.6	$ 4,200.3
% of Total Fair Value	8.20%	7.90%	9.20%
Total fixed maturities \| Redeemable preferred stocks
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 171	$ 181.2	$ 181.3
Gross Unrealized Gains	1.3	3.6	2.5
Gross Unrealized Losses	(0.7)	(1.4)	(3.2)
Net Holding Period Gains (Losses)	13	11.3	2.4
Total available-for-sale securities	$ 184.6	$ 194.7	$ 183
% of Total Fair Value	0.40%	0.40%	0.40%
Short-term investments
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 1,088.7	$ 5,218.5	$ 4,667.8
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 1,088.7	$ 5,218.5	$ 4,667.8
% of Total Fair Value	2.10%	11.00%	10.20%

Investments Hybrid Securities (

Investments Hybrid Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Hybrid Instruments [Line Items]
Hybrid securities	$ 818	$ 354.6	$ 242.2
Corporate debt securities
Hybrid Instruments [Line Items]
Hybrid securities	363.3	188.4	101
Residential mortgage-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	217	0	0
Other asset-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	104.5	34.8	18.7
Redeemable preferred stocks
Hybrid Instruments [Line Items]
Hybrid securities	$ 133.2	$ 131.4	$ 122.5

Investments Composition of Fixe

Investments Composition of Fixed Maturities by Maturity (Detail) - Total fixed maturities $ in Millions	Sep. 30, 2021USD ($)
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract]
Less than one year	$ 6,124.5
One to five years	27,606.8
Five to ten years	10,761.3
Ten years or greater	63.4
Cost	44,556
Fair Value
Less than one year	6,153.2
One to five years	27,968.5
Five to ten years	10,861
Ten years or greater	63
Fair Value	$ 45,045.7

Investments Gross Unrealized lo

Investments Gross Unrealized losses by Major Security (Detail) $ in Millions	Sep. 30, 2021USD ($)security	Dec. 31, 2020USD ($)security	Sep. 30, 2020USD ($)security
Number of Securities
Total \| security	644	133	214
Less than 12 Months \| security	598	64	152
12 Months or Greater \| security	46	69	62
Fair Value
Total	$ 19,190.2	$ 2,981.5	$ 4,934
Less than 12 Months	17,632.9	2,017.1	4,207.3
12 Months or Greater	1,557.3	964.4	726.7
Unrealized Losses
Total	(138.1)	(17.3)	(40.5)
Less than 12 Months	(108.2)	(4.9)	(26.1)
12 Months or Greater	$ (29.9)	$ (12.4)	$ (14.4)
U.S. government obligations
Number of Securities
Total \| security	73	9	13
Less than 12 Months \| security	65	9	13
12 Months or Greater \| security	8	0	0
Fair Value
Total	$ 13,352.8	$ 1,511	$ 1,993.8
Less than 12 Months	12,152.6	1,511	1,993.8
12 Months or Greater	1,200.2	0	0
Unrealized Losses
Total	(95.9)	(3.7)	(1.6)
Less than 12 Months	(70.1)	(3.7)	(1.6)
12 Months or Greater	$ (25.8)	$ 0	$ 0
State and local government obligations
Number of Securities
Total \| security	96	30	45
Less than 12 Months \| security	81	30	45
12 Months or Greater \| security	15	0	0
Fair Value
Total	$ 710.3	$ 208.7	$ 320.1
Less than 12 Months	635.9	208.7	320.1
12 Months or Greater	74.4	0	0
Unrealized Losses
Total	(6.7)	(0.7)	(0.5)
Less than 12 Months	(4.9)	(0.7)	(0.5)
12 Months or Greater	$ (1.8)	$ 0	$ 0
Corporate debt securities
Number of Securities
Total \| security	227	7	28
Less than 12 Months \| security	225	7	28
12 Months or Greater \| security	2	0	0
Fair Value
Total	$ 1,581.3	$ 129.4	$ 537.8
Less than 12 Months	1,551.5	129.4	537.8
12 Months or Greater	29.8	0	0
Unrealized Losses
Total	(10.5)	(0.1)	(3.6)
Less than 12 Months	(10)	(0.1)	(3.6)
12 Months or Greater	$ (0.5)	$ 0	$ 0
Residential mortgage-backed securities
Number of Securities
Total \| security	63	21	28
Less than 12 Months \| security	53	0	9
12 Months or Greater \| security	10	21	19
Fair Value
Total	$ 149.3	$ 44.4	$ 86.7
Less than 12 Months	135.1	0	38.7
12 Months or Greater	14.2	44.4	48
Unrealized Losses
Total	(0.9)	(0.9)	(1.4)
Less than 12 Months	(0.6)	0	(1)
12 Months or Greater	$ (0.3)	$ (0.9)	$ (0.4)
Commercial mortgage-backed securities
Number of Securities
Total \| security	70	43	77
Less than 12 Months \| security	65	9	46
12 Months or Greater \| security	5	34	31
Fair Value
Total	$ 1,758.8	$ 893.3	$ 1,794.8
Less than 12 Months	1,543.6	93.6	1,170.4
12 Months or Greater	215.2	799.7	624.4
Unrealized Losses
Total	(20.6)	(10)	(27.9)
Less than 12 Months	(19.9)	(0.3)	(17.3)
12 Months or Greater	$ (0.7)	$ (9.7)	$ (10.6)
Other asset-backed securities
Number of Securities
Total \| security	114	22	22
Less than 12 Months \| security	109	9	11
12 Months or Greater \| security	5	13	11
Fair Value
Total	$ 1,625.9	$ 183.7	$ 191.5
Less than 12 Months	1,614.2	74.4	146.5
12 Months or Greater	11.7	109.3	45
Unrealized Losses
Total	(2.8)	(0.5)	(2.3)
Less than 12 Months	(2.7)	(0.1)	(2.1)
12 Months or Greater	$ (0.1)	$ (0.4)	$ (0.2)
Redeemable preferred stocks
Number of Securities
Total \| security	1	1	1
Less than 12 Months \| security	0	0	0
12 Months or Greater \| security	1	1	1
Fair Value
Total	$ 11.8	$ 11	$ 9.3
Less than 12 Months	0	0	0
12 Months or Greater	11.8	11	9.3
Unrealized Losses
Total	(0.7)	(1.4)	(3.2)
Less than 12 Months	0	0	0
12 Months or Greater	$ (0.7)	$ (1.4)	$ (3.2)

Investments Components of Net R

Investments Components of Net Realized Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	$ 50.9	$ 198.1	$ 275.7	$ 765.8
Equity Securities, FV-NI, Gross Realized Gains on Sales	8.5	2.3	378.3	97.1
Debt and Equity Securities, Gross Realized Gains on Sales	59.4	200.4	654	862.9
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	(3.5)	(4.4)	(39.9)	(24)
Equity Securities, FV-NI, Gross Realized Losses on Sales	(2)	(0.1)	(6.3)	(67.8)
Debt and Equity Securities, Gross Realized Losses on Sales	(5.5)	(4.5)	(46.2)	(91.8)
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	47.4	193.7	235.8	741.8
Equity Securities, FV-NI, Net Realized Gain (Loss) on Sales	6.5	2.2	372	29.3
Gross Realized Gain (Loss), excluding Holding Period and OTTI	53.9	195.9	607.8	771.1
Unrealized Gain (Loss) on Hybrid Instrument, Net	1	3.2	0	(3.2)
Net holding period gains (losses), equity securities	(16.9)	333.5	479.8	101.9
Net Holding Period Realized Gain (Loss)	(15.9)	336.7	479.8	98.7
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Other Assets	1.2	0	3.7	0
Total net realized gains (losses) on securities	36.8	532.6	1,083.9	869.8
Other Assets
Gain (Loss) on Securities [Line Items]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Other Assets	1.2	0	3.7	0
U.S. government obligations
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	19.6	90.2	97.5	565.8
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	(2)	(1.5)	(28.5)	(4.8)
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	17.6	88.7	69	561
State and local government obligations
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	6	43.9	50.3	58.9
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	0	0	(3)	0
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	6	43.9	47.3	58.9
Corporate debt securities
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	23.6	58.8	84.4	125.5
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	(0.7)	0	(6.1)	(6.5)
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	22.9	58.8	78.3	119
Residential mortgage-backed securities
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	0	0	0.3	0
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	(0.3)	0	(0.3)	0
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	(0.3)	0	0	0
Commercial mortgage-backed securities
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	1.4	5	40.7	15.4
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	(0.5)	(2.9)	(1.6)	(12.7)
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	0.9	2.1	39.1	2.7
Other asset-backed securities
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	0.3	0.2	1	0.2
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	0	0	(0.4)	0
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	0.3	0.2	0.6	0.2
Redeemable preferred stocks
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	0	0	1.5	0
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	0	0	1.5	0
Nonredeemable preferred stocks
Gain (Loss) on Securities [Line Items]
Equity Securities, FV-NI, Gross Realized Gains on Sales	2.3	0.3	26	20
Equity Securities, FV-NI, Gross Realized Losses on Sales	(0.3)	(0.1)	(0.7)	(7.5)
Equity Securities, FV-NI, Net Realized Gain (Loss) on Sales	2	0.2	25.3	12.5
Common equities
Gain (Loss) on Securities [Line Items]
Equity Securities, FV-NI, Gross Realized Gains on Sales	6.2	2	352.3	77.1
Equity Securities, FV-NI, Gross Realized Losses on Sales	(1.7)	0	(5.6)	(60.3)
Equity Securities, FV-NI, Net Realized Gain (Loss) on Sales	$ 4.5	$ 2	$ 346.7	$ 16.8

Investments Components of Equit

Investments Components of Equity Realized Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Realized Investment Gains (Losses)	$ (10.4)	$ 335.7	$ 851.8	$ 131.2
Equity Securities, FV-NI, Net Realized Gain (Loss) on Sales	6.5	2.2	372	29.3
Equity Securities, FV-NI, Unrealized Gain (Loss)	$ (16.9)	$ 333.5	$ 479.8	$ 101.9

Investments Components of Net I

Investments Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net Investment Income [Line Items]
Gross investment income	$ 208.9	$ 230.5	$ 639.8	$ 715.5
Investment expenses	(6.6)	(4.7)	(18.5)	(14.5)
Net Investment Income	202.3	225.8	621.3	701
Available-for-sale Securities
Net Investment Income [Line Items]
Gross investment income	175.9	201.2	541.4	629.4
Total equity securities
Net Investment Income [Line Items]
Gross investment income	33	29.3	98.4	86.1
Total equity securities \| Nonredeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	17.3	15.8	52.5	44.3
Total equity securities \| Common equities
Net Investment Income [Line Items]
Gross investment income	15.7	13.5	45.9	41.8
Total fixed maturities
Net Investment Income [Line Items]
Gross investment income	175.5	195	538.8	602.5
Total fixed maturities \| U.S. government obligations
Net Investment Income [Line Items]
Gross investment income	39	36.7	106.8	135
Total fixed maturities \| State and local government obligations
Net Investment Income [Line Items]
Gross investment income	9.7	18.5	34.6	44.2
Total fixed maturities \| Corporate debt securities
Net Investment Income [Line Items]
Gross investment income	70.2	76.6	228.6	211.2
Total fixed maturities \| Residential mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	3.2	0.8	9.4	8.8
Total fixed maturities \| Commercial mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	35	37.3	105.2	114.2
Total fixed maturities \| Other asset-backed securities
Net Investment Income [Line Items]
Gross investment income	16.1	22.6	47.1	76.6
Total fixed maturities \| Redeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	2.3	2.5	7.1	12.5
Short-term investments
Net Investment Income [Line Items]
Gross investment income	$ 0.4	$ 6.2	$ 2.6	$ 26.9

Investments - Additional Inform

Investments - Additional Information (Detail)	3 Months Ended	9 Months Ended
Investments - Additional Information (Detail)	Sep. 30, 2021USD ($)security	Sep. 30, 2021USD ($)security	Dec. 31, 2020USD ($)	Sep. 30, 2020USD ($)
Schedule of Debt and Equity Securities [Line Items]
Principal amount of bonds and certificates of deposit	$ 407,800,000	$ 407,800,000
Securities Exceeding 10% of Shareholders' Equity	0	0	$ 0	$ 0
Non-Income Producing Securities	0	0
Total available-for-sale securities	46,134,400,000	46,134,400,000	42,029,400,000	40,981,300,000
Open reverse repurchase commitments	0	0	0	0
Securities Sold under Agreements to Repurchase	$ 0	$ 0	0	0
Debt Securities, Available-for-sale, Unrealized Loss Position, Positions Downgraded \| security	2	2
Combined fair value of securities	$ 10,500,000	$ 10,500,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Downgrades in Period	400,000	400,000
Debt Securities, Available-for-sale, Allowance for Credit Loss, Not Previously Recorded		0
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 0	0	$ 0	$ 0
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff		$ 0
Percent Change in Investment Income	9.00%	11.00%
Percent Change in Book Yield	21.00%	23.00%
Foreign Government Debt
Schedule of Debt and Equity Securities [Line Items]
Total available-for-sale securities	$ 12,300,000	$ 12,300,000

Fair Value Composition of Inves

Fair Value Composition of Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	$ 52,287.4	$ 47,530.3	$ 45,764.3
Fair Value	5,881.7	6,793.5	6,687.4
Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	46,134.4	42,029.4	40,981.3
Available-for-sale Securities \| Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	45,045.7	36,810.9	36,313.5
Available-for-sale Securities \| Total fixed maturities \| Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	34,182	26,147	25,345.6
Available-for-sale Securities \| Total fixed maturities \| Debt Securities \| U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	21,142.1	12,740	10,203.8
Available-for-sale Securities \| Total fixed maturities \| Debt Securities \| State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,165.9	3,221.8	4,529.8
Available-for-sale Securities \| Total fixed maturities \| Debt Securities \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10,874	10,185.2	10,612
Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10,679.1	10,469.2	10,784.9
Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities \| Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	627.9	509.5	531
Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities \| Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	5,789	6,175.1	6,053.6
Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities \| Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,262.2	3,784.6	4,200.3
Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	184.6	194.7	183
Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks \| Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	51.4	51.6	49.2
Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks \| Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	11.7	11.3
Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	133.2	131.4	122.5
Available-for-sale Securities \| Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,088.7	5,218.5	4,667.8
Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	6,153	5,500.9	4,783
Total equity securities \| Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,572.8	1,422.9	1,298.2
Total equity securities \| Nonredeemable preferred stocks \| Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,470.8	1,340	1,221.8
Total equity securities \| Nonredeemable preferred stocks \| Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	42.1	41.9	40.7
Total equity securities \| Nonredeemable preferred stocks \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	59.9	41	35.7
Total equity securities \| Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,580.2	4,078	3,484.8
Total equity securities \| Common equities \| Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,567.6	4,074.9	3,482.1
Total equity securities \| Common equities \| Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	12.6	3.1	2.7
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	26,834.1	21,623.3	18,289.8
Fair Value	0	0	0
Fair Value, Inputs, Level 1 \| Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	22,231.2	17,455.7	14,720.1
Fair Value, Inputs, Level 1 \| Available-for-sale Securities \| Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	21,152.7	12,750.8	10,213.9
Fair Value, Inputs, Level 1 \| Available-for-sale Securities \| Total fixed maturities \| Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	21,142.1	12,740	10,203.8
Fair Value, Inputs, Level 1 \| Available-for-sale Securities \| Total fixed maturities \| Debt Securities \| U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	21,142.1	12,740	10,203.8
Fair Value, Inputs, Level 1 \| Available-for-sale Securities \| Total fixed maturities \| Debt Securities \| State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 \| Available-for-sale Securities \| Total fixed maturities \| Debt Securities \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 \| Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 \| Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities \| Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 \| Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities \| Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 \| Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities \| Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 \| Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10.6	10.8	10.1
Fair Value, Inputs, Level 1 \| Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks \| Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 \| Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks \| Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 \| Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10.6	10.8	10.1
Fair Value, Inputs, Level 1 \| Available-for-sale Securities \| Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,078.5	4,704.9	4,506.2
Fair Value, Inputs, Level 1 \| Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,602.9	4,167.6	3,569.7
Fair Value, Inputs, Level 1 \| Total equity securities \| Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	89.8	117.7	112.6
Fair Value, Inputs, Level 1 \| Total equity securities \| Nonredeemable preferred stocks \| Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	89.8	117.7	112.6
Fair Value, Inputs, Level 1 \| Total equity securities \| Nonredeemable preferred stocks \| Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 \| Total equity securities \| Nonredeemable preferred stocks \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 \| Total equity securities \| Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,513.1	4,049.9	3,457.1
Fair Value, Inputs, Level 1 \| Total equity securities \| Common equities \| Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,513.1	4,049.9	3,457.1
Fair Value, Inputs, Level 1 \| Total equity securities \| Common equities \| Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	25,275.4	25,852.2	27,418.5
Fair Value	5,881.7	6,793.5	6,687.4
Fair Value, Inputs, Level 2 \| Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	23,903.2	24,573.7	26,261.2
Fair Value, Inputs, Level 2 \| Available-for-sale Securities \| Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	23,893	24,060.1	26,099.6
Fair Value, Inputs, Level 2 \| Available-for-sale Securities \| Total fixed maturities \| Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	13,039.9	13,407	15,141.8
Fair Value, Inputs, Level 2 \| Available-for-sale Securities \| Total fixed maturities \| Debt Securities \| U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 2 \| Available-for-sale Securities \| Total fixed maturities \| Debt Securities \| State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,165.9	3,221.8	4,529.8
Fair Value, Inputs, Level 2 \| Available-for-sale Securities \| Total fixed maturities \| Debt Securities \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10,874	10,185.2	10,612
Fair Value, Inputs, Level 2 \| Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10,679.1	10,469.2	10,784.9
Fair Value, Inputs, Level 2 \| Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities \| Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	627.9	509.5	531
Fair Value, Inputs, Level 2 \| Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities \| Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	5,789	6,175.1	6,053.6
Fair Value, Inputs, Level 2 \| Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities \| Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,262.2	3,784.6	4,200.3
Fair Value, Inputs, Level 2 \| Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	174	183.9	172.9
Fair Value, Inputs, Level 2 \| Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks \| Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	51.4	51.6	49.2
Fair Value, Inputs, Level 2 \| Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks \| Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	11.7	11.3
Fair Value, Inputs, Level 2 \| Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	122.6	120.6	112.4
Fair Value, Inputs, Level 2 \| Available-for-sale Securities \| Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10.2	513.6	161.6
Fair Value, Inputs, Level 2 \| Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,372.2	1,278.5	1,157.3
Fair Value, Inputs, Level 2 \| Total equity securities \| Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,372.2	1,278.5	1,157.3
Fair Value, Inputs, Level 2 \| Total equity securities \| Nonredeemable preferred stocks \| Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,304.6	1,212.3	1,096.1
Fair Value, Inputs, Level 2 \| Total equity securities \| Nonredeemable preferred stocks \| Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	42.1	41.9	40.7
Fair Value, Inputs, Level 2 \| Total equity securities \| Nonredeemable preferred stocks \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	25.5	24.3	20.5
Fair Value, Inputs, Level 2 \| Total equity securities \| Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 2 \| Total equity securities \| Common equities \| Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 2 \| Total equity securities \| Common equities \| Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	177.9	54.8	56
Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Available-for-sale Securities \| Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Available-for-sale Securities \| Total fixed maturities \| Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Available-for-sale Securities \| Total fixed maturities \| Debt Securities \| U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Available-for-sale Securities \| Total fixed maturities \| Debt Securities \| State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Available-for-sale Securities \| Total fixed maturities \| Debt Securities \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities \| Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities \| Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities \| Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks \| Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks \| Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Available-for-sale Securities \| Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	177.9	54.8	56
Fair Value, Inputs, Level 3 \| Total equity securities \| Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	110.8	26.7	28.3
Fair Value, Inputs, Level 3 \| Total equity securities \| Nonredeemable preferred stocks \| Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	76.4	10	13.1
Fair Value, Inputs, Level 3 \| Total equity securities \| Nonredeemable preferred stocks \| Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 \| Total equity securities \| Nonredeemable preferred stocks \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	34.4	16.7	15.2
Fair Value, Inputs, Level 3 \| Total equity securities \| Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	67.1	28.1	27.7
Fair Value, Inputs, Level 3 \| Total equity securities \| Common equities \| Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	54.5	25	25
Fair Value, Inputs, Level 3 \| Total equity securities \| Common equities \| Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	12.6	3.1	2.7
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	48,363	43,353.6	42,183.3
Long-term Debt	4,898.2	5,396.1	5,395.4
Carrying (Reported) Amount, Fair Value Disclosure \| Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	45,644.7	40,807.6	39,750.6
Carrying (Reported) Amount, Fair Value Disclosure \| Available-for-sale Securities \| Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	44,556	35,589.1	35,082.8
Carrying (Reported) Amount, Fair Value Disclosure \| Available-for-sale Securities \| Total fixed maturities \| Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	33,785.6	25,117	24,255.9
Carrying (Reported) Amount, Fair Value Disclosure \| Available-for-sale Securities \| Total fixed maturities \| Debt Securities \| U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	21,093.1	12,437.9	9,835.1
Carrying (Reported) Amount, Fair Value Disclosure \| Available-for-sale Securities \| Total fixed maturities \| Debt Securities \| State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,123.8	3,099.4	4,382.1
Carrying (Reported) Amount, Fair Value Disclosure \| Available-for-sale Securities \| Total fixed maturities \| Debt Securities \| Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10,568.7	9,579.7	10,038.7
Carrying (Reported) Amount, Fair Value Disclosure \| Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10,599.4	10,290.9	10,645.6
Carrying (Reported) Amount, Fair Value Disclosure \| Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities \| Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	624.4	503.3	525.4
Carrying (Reported) Amount, Fair Value Disclosure \| Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities \| Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	5,739.4	6,042.6	5,964.4
Carrying (Reported) Amount, Fair Value Disclosure \| Available-for-sale Securities \| Total fixed maturities \| Asset-backed Securities \| Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,235.6	3,745	4,155.8
Carrying (Reported) Amount, Fair Value Disclosure \| Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	171	181.2	181.3
Carrying (Reported) Amount, Fair Value Disclosure \| Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks \| Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	50.8	51.1	51.2
Carrying (Reported) Amount, Fair Value Disclosure \| Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks \| Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	10	10
Carrying (Reported) Amount, Fair Value Disclosure \| Available-for-sale Securities \| Total fixed maturities \| Redeemable preferred stocks \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	120.2	120.1	120.1
Carrying (Reported) Amount, Fair Value Disclosure \| Available-for-sale Securities \| Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,088.7	5,218.5	4,667.8
Carrying (Reported) Amount, Fair Value Disclosure \| Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,718.3	2,546	2,432.7
Carrying (Reported) Amount, Fair Value Disclosure \| Total equity securities \| Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,479.9	1,333.7	1,277.7
Carrying (Reported) Amount, Fair Value Disclosure \| Total equity securities \| Nonredeemable preferred stocks \| Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,399.8	1,253.6	1,197.7
Carrying (Reported) Amount, Fair Value Disclosure \| Total equity securities \| Nonredeemable preferred stocks \| Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	40	40	39.9
Carrying (Reported) Amount, Fair Value Disclosure \| Total equity securities \| Nonredeemable preferred stocks \| Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	40.1	40.1	40.1
Carrying (Reported) Amount, Fair Value Disclosure \| Total equity securities \| Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,238.4	1,212.3	1,155
Carrying (Reported) Amount, Fair Value Disclosure \| Total equity securities \| Common equities \| Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,225.8	1,209.2	1,152.3
Carrying (Reported) Amount, Fair Value Disclosure \| Total equity securities \| Common equities \| Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	$ 12.6	$ 3.1	$ 2.7

Fair Value Summary of Changes i

Fair Value Summary of Changes in Fair Value Associated with Level 3 Assets (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 174.9	$ 53.6	$ 54.8	$ 43.4
Calls/ Maturities/ Paydowns	0	0	0	0
Purchases	3	2.4	71.7	27.4
Sales	0	0	0	0
Net Realized (Gain)/Loss on Sales	0	0	(4.5)	0
Change in Valuation	0	0	55.9	(14.8)
Net Transfers In (Out)	0	0	0	0
Fair value, ending balance	177.9	56	177.9	56
Total equity securities \| Financials \| Nonredeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	76.4	38.1	10	27.1
Calls/ Maturities/ Paydowns	0	(25)	0	(25)
Purchases	0	0	60.2	25
Sales	0	0	0	0
Net Realized (Gain)/Loss on Sales	0	0	0	0
Change in Valuation	0	0	6.2	(14)
Net Transfers In (Out)	0	0	0	0
Fair value, ending balance	76.4	13.1	76.4	13.1
Total equity securities \| Industrials \| Nonredeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	34.4	15.2	16.7	16
Calls/ Maturities/ Paydowns	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Net Realized (Gain)/Loss on Sales	0	0	(4.5)	0
Change in Valuation	0	0	22.2	(0.8)
Net Transfers In (Out)	0	0	0	0
Fair value, ending balance	34.4	15.2	34.4	15.2
Total equity securities \| Common Stocks, excluding Other Risk \| Common equities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	52.5	0	25	0
Calls/ Maturities/ Paydowns	0	25	0	25
Purchases	2	0	2	0
Sales	0	0	0	0
Net Realized (Gain)/Loss on Sales	0	0	0	0
Change in Valuation	0	0	27.5	0
Net Transfers In (Out)	0	0	0	0
Fair value, ending balance	54.5	25	54.5	25
Total equity securities \| Other risk investments \| Common equities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	11.6	0.3	3.1	0.3
Calls/ Maturities/ Paydowns	0	0	0	0
Purchases	1	2.4	9.5	2.4
Sales	0	0	0	0
Net Realized (Gain)/Loss on Sales	0	0	0	0
Change in Valuation	0	0	0	0
Net Transfers In (Out)	0	0	0	0
Fair value, ending balance	$ 12.6	$ 2.7	$ 12.6	$ 2.7

Fair Value Summary of Quantitat

Fair Value Summary of Quantitative Information about Level 3 Fair Value Measurements (Details) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 52,287.4	$ 45,764.3	$ 47,530.3
Fair Value, Inputs, Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	177.9	56	54.8
Fair Value, Inputs, Level 3 \| Total Quantitative Level Three Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	165.3	53.3	51.7
Fair Value, Inputs, Level 3 \| Third Party Pricing Exemption Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	12.6	2.7	3.1
Fair Value, Inputs, Level 3 \| Nonredeemable preferred stocks \| Financials \| Recent Transaction Price, Twenty Seven Point Five Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 16.2
Fair Value Inputs, Recent Transaction Price	$ 27.5
Fair Value, Inputs, Level 3 \| Nonredeemable preferred stocks \| Financials \| Unadjusted Purchase Price, Twenty Point One Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 22.5
Fair Value Inputs, Original Issue Price	$ 20.1
Fair Value, Inputs, Level 3 \| Nonredeemable preferred stocks \| Financials \| Unadjusted Purchase Price, Eleven Point Eight Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 12.5
Fair Value Inputs, Original Issue Price	$ 11.8
Fair Value, Inputs, Level 3 \| Nonredeemable preferred stocks \| Financials \| Unadjusted Purchase Price, Thirty Two Point One Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 10.2
Fair Value Inputs, Original Issue Price	$ 32.1
Fair Value, Inputs, Level 3 \| Nonredeemable preferred stocks \| Financials \| Unadjusted Purchase Price, Six Point Eight Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 15
Fair Value Inputs, Original Issue Price	$ 6.8
Fair Value, Inputs, Level 3 \| Nonredeemable preferred stocks \| Financials \| Guideline Public Company Method, Four Point Four Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value		$ 13.1
Fair Value Inputs, Guideline Public Company Method		$ 4.4
Fair Value, Inputs, Level 3 \| Nonredeemable preferred stocks \| Financials \| Unadjusted Purchase Price, Sixteen Point Nine Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value			$ 10
Fair Value Inputs, Original Issue Price			$ 16.9
Fair Value, Inputs, Level 3 \| Nonredeemable preferred stocks \| Industrials \| Recent Transaction Price, Five Point Two Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 10.7
Fair Value Inputs, Recent Transaction Price	$ 5.2
Fair Value, Inputs, Level 3 \| Nonredeemable preferred stocks \| Industrials \| Recent Transaction Price, Nine Point Six Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 23.7
Fair Value Inputs, Recent Transaction Price	$ 9.6
Fair Value, Inputs, Level 3 \| Nonredeemable preferred stocks \| Industrials \| Guideline Public Company Method, Five Point Six Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value		$ 7.1
Fair Value Inputs, Guideline Public Company Method		$ 5.6
Fair Value, Inputs, Level 3 \| Nonredeemable preferred stocks \| Industrials \| Guideline Public Company Method, Four Point Zero Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value		$ 8.1
Fair Value Inputs, Guideline Public Company Method		$ 4
Fair Value, Inputs, Level 3 \| Nonredeemable preferred stocks \| Industrials \| Guideline Public Company Method, Five Point Four Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value			$ 6.9
Fair Value Inputs, Guideline Public Company Method			$ 5.4
Fair Value, Inputs, Level 3 \| Nonredeemable preferred stocks \| Industrials \| Guideline Public Company Method, Four Point Eight Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value			$ 9.8
Fair Value Inputs, Guideline Public Company Method			$ 4.8
Fair Value, Inputs, Level 3 \| Common equities \| Guideline Public Company Method, Seven Point Seven Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 54.5
Fair Value Inputs, Recent Transaction Price	$ 7.7
Fair Value, Inputs, Level 3 \| Common equities \| Unadjusted Purchase Price, Three Point Seven Dollars per Share
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value		$ 25	$ 25
Fair Value Inputs, Original Issue Price		$ 3.7	$ 3.7

Fair Value - Additional Informa

Fair Value - Additional Information (Detail)	Sep. 30, 2021USD ($)security	Dec. 31, 2020USD ($)security	Sep. 30, 2020USD ($)security
Fair Value Measurements [Line Items]
Investments, Fair Value	$ 52,287,400,000	$ 47,530,300,000	$ 45,764,300,000
Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	82.00%	76.00%	75.00%
Investments, Fair Value	$ 26,834,100,000	$ 21,623,300,000	$ 18,289,800,000
Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	97.00%	99.00%	99.00%
Dealer Quoted Prices, Percent of FV Hierarchy Level	3.00%	1.00%	1.00%
Investments, Fair Value	$ 25,275,400,000	$ 25,852,200,000	$ 27,418,500,000
Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value	177,900,000	54,800,000	56,000,000
Total fixed maturities \| Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value	$ 0	$ 0	$ 0
Nonredeemable preferred stocks \| Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Number of securities \| security	7	3	3
Common equities \| Fair Value, Inputs, Level 3 \| Other risk investments
Fair Value Measurements [Line Items]
Number of securities \| security	6	2	2
Common equities \| Fair Value, Inputs, Level 3 \| Common Stocks, excluding Other Risk
Fair Value Measurements [Line Items]
Number of securities \| security	1	1	1

Debt (Detail)

Debt (Detail) - USD ($)		Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Debt Instrument [Line Items]
Fair Value		$ 5,881,700,000	$ 6,793,500,000	$ 6,687,400,000
Debt, Long-term and Short-term, Combined Amount	[1]	4,898,200,000	5,396,100,000	5,395,400,000
3.75% Senior Notes due 2021 (issued: $500.0, August 2011)
Debt Instrument [Line Items]
Long-term Debt		0	499,800,000	499,700,000
Fair Value		$ 0	510,900,000	515,300,000
Debt, stated interest rate		3.75%
Debt, face amount		$ 500,000,000	500,000,000	500,000,000
2.45% Senior Notes due 2027 (issued: $500.0, August 2016)
Debt Instrument [Line Items]
Long-term Debt		497,600,000	497,300,000	497,200,000
Fair Value		$ 530,600,000	541,100,000	532,000,000
Debt, stated interest rate		2.45%
Debt, face amount		$ 500,000,000	500,000,000	500,000,000
6 5/8% Senior Notes due 2029 (issued: $300.0, March 1999)
Debt Instrument [Line Items]
Long-term Debt		297,100,000	296,900,000	296,800,000
Fair Value		$ 390,900,000	409,400,000	411,400,000
Debt, stated interest rate		6.625%
Debt, face amount		$ 300,000,000	300,000,000	300,000,000
4.00% Senior Notes due 2029 (issued: $550.0, October 2018)
Debt Instrument [Line Items]
Long-term Debt		545,800,000	545,500,000	545,300,000
Fair Value		$ 627,900,000	660,400,000	662,500,000
Debt, stated interest rate		4.00%
Debt, face amount		$ 550,000,000	550,000,000	550,000,000
3.20% Senior Notes due 2030 (issued: $500.0, March 2020)
Debt Instrument [Line Items]
Long-term Debt		496,400,000	496,100,000	496,000,000
Fair Value		$ 543,200,000	575,500,000	572,400,000
Debt, stated interest rate		3.20%
Debt, face amount		$ 500,000,000	500,000,000	500,000,000
6.25% Senior Notes due 2032 (issued: $400.0, November 2002)
Debt Instrument [Line Items]
Long-term Debt		396,100,000	396,000,000	395,900,000
Fair Value		$ 545,500,000	582,000,000	587,200,000
Debt, stated interest rate		6.25%
Debt, face amount		$ 400,000,000	400,000,000	400,000,000
4.35% Senior Notes due 2044 (issued: $350.0, April 2014)
Debt Instrument [Line Items]
Long-term Debt		346,800,000	346,700,000	346,700,000
Fair Value		$ 433,600,000	459,700,000	450,500,000
Debt, stated interest rate		4.35%
Debt, face amount		$ 350,000,000	350,000,000	350,000,000
3.70% Senior Notes due 2045 (issued: $400.0, January 2015)
Debt Instrument [Line Items]
Long-term Debt		395,600,000	395,500,000	395,500,000
Fair Value		$ 456,200,000	481,000,000	473,500,000
Debt, stated interest rate		3.70%
Debt, face amount		$ 400,000,000	400,000,000	400,000,000
4.125% Senior Notes due 2047 (issued: $850.0, April 2017)
Debt Instrument [Line Items]
Long-term Debt		841,900,000	841,700,000	841,700,000
Fair Value		$ 1,018,500,000	1,113,100,000	1,078,500,000
Debt, stated interest rate		4.125%
Debt, face amount		$ 850,000,000	850,000,000	850,000,000
4.20% Senior Notes due 2048 (issued: $600.0, March 2018)
Debt Instrument [Line Items]
Long-term Debt		590,200,000	590,000,000	590,000,000
Fair Value		$ 736,300,000	806,700,000	775,300,000
Debt, stated interest rate		4.20%
Debt, face amount		$ 600,000,000	600,000,000	600,000,000
3.95% Senior Notes due 2050 (issued: $500.0, March 2020)
Debt Instrument [Line Items]
Long-term Debt		490,700,000	490,600,000	490,600,000
Fair Value		$ 599,000,000	653,700,000	628,800,000
Debt, stated interest rate		3.95%
Debt, face amount		$ 500,000,000	$ 500,000,000	$ 500,000,000

[1]	Consists of long-term debt. See Note 4 – Debt for further discussion.

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($)	9 Months Ended		12 Months Ended
Debt - Additional Information (Detail) - USD ($)	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Debt Instrument [Line Items]
Long-Term Debt, Maturity, Remainder of Fiscal Year	$ 0	$ 500,000,000	$ 500,000,000
Repayments of Long-term Debt	$ 500,000,000	0
3.75% Senior Notes due 2021 (issued: $500.0, August 2011)
Debt Instrument [Line Items]
Debt, stated interest rate	3.75%
Discretionary Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 250,000,000
Borrowings outstanding	$ 0	$ 0	$ 0

Income Taxes Additional Informa

Income Taxes Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
Income Taxes Additional Information (Details) - USD ($)	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Income Taxes [Line Items]
Valuation allowance	$ 0	$ 0	$ 0	$ 0	$ 0
Effective Income Tax Rate Reconciliation, Percent	14.70%	20.40%	20.50%	20.70%

Loss and Loss Adjustment Expe_3

Loss and Loss Adjustment Expense Reserves (Details) - USD ($) $ in Millions	Jan. 01, 2021	Jan. 01, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves			$ 25,926.3	$ 19,589.2	$ 20,265.8	$ 18,105.4
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments and Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item			4,822.5	3,804.3	3,798.2	3,212.2
Liability for Unpaid Claims and Claims Adjustment Expense, Net	$ 17,200.1	$ 14,893.2	21,103.8	15,784.9	$ 16,467.6	$ 14,893.2
Liability for Unpaid Claims and Claims Adjustment Expense, Business Acquisitions	$ 732.5	$ 0
Current Year Claims and Claims Adjustment Expense			24,610.8	18,073.6
Prior Year Claims and Claims Adjustment Expense			(156.8)	(116.1)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims			24,767.6	18,189.7
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year			13,650.6	10,615.2
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years			7,213.3	6,682.8
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid			20,863.9	$ 17,298
Short-Duration Insurance Contract, Accident Year 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			(109)
Short-duration Insurance Contracts, Accident Year 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of Prior Year Claims and Claims Adjustment Expense				80.00%
Short Duration Insurance Contracts Accident Year 2018 and Prior Member
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			(113)
Personal Lines \| Personal auto businesses
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			(48)	$ (47)
Personal Lines \| Special Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			12	22
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			(91)	(104)
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			$ (31)	$ 13
Agency \| Personal Lines \| Personal auto businesses
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of Prior Year Claims and Claims Adjustment Expense			90.00%

Supplemental Cash Flow Inform_3

Supplemental Cash Flow Information (Detail) - USD ($)	9 Months Ended
Supplemental Cash Flow Information (Detail) - USD ($)	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Open reverse repurchase commitments	$ 0	$ 0			$ 0
Dividends payable on common shares	58,500,000	58,600,000			2,694,500,000
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	79,500,000	48,500,000
Operating Lease, Payments	67,800,000	66,600,000
Restricted Cash and Cash Equivalents	14,900,000	1,000,000			0
Income Taxes Paid	669,500,000	1,079,200,000
Interest Paid, Excluding Capitalized Interest, Operating Activities	186,300,000	168,400,000
Collateral Against Unpaid Premiums
Restricted Cash and Cash Equivalents	14,400,000
Cash and Cash Equivalents
Open reverse repurchase commitments	134,600,000	77,200,000			93,500,000
Common equities \| Quarterly dividend [Member]
Dividends payable on common shares	$ 58,500,000	$ 58,600,000	$ 58,500,000	$ 58,500,000	$ 58,600,000	$ 58,500,000	$ 58,500,000	$ 58,500,000

Segment Information Operating R

Segment Information Operating Results (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Revenues	$ 11,859.2	$ 10,947.2	$ 35,209.9	$ 31,242.3
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	139	1,923.5	3,006.1	5,067
Amortization of intangible assets	14.6	14.2	43.1	42.8
Personal Lines \| Agency \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	4,267.9	4,001.6	12,586.4	11,749.3
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	41	596.5	796	1,748.7
Personal Lines \| Direct \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	4,690.2	4,309.8	13,755.8	12,470.1
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(62.4)	517.6	480.6	1,637.8
Operating Segments \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	11,364.8	9,973.5	32,767.3	29,052.8
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(46.9)	1,217.1	1,455.8	3,642.3
Operating Segments \| Personal Lines \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	8,958.1	8,311.4	26,342.2	24,219.4
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	$ (21.4)	$ 1,114.1	$ 1,276.6	$ 3,386.5
Personal auto insurance percentage of the total personal lines segment net premiums earned	94.00%	94.00%	94.00%	94.00%
Operating Segments \| Commercial Lines \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	$ 1,877.4	$ 1,214.8	$ 4,917	$ 3,532.8
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	197.5	155.9	556.1	448.2
Operating Segments \| Property \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	526.5	447.3	1,501.3	1,300.6
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(222.7)	(52.9)	(376.7)	(192.4)
Amortization of intangible assets	14.2	14.2	42.5	42.8
Operating Segments \| Other Indemnity \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	2.8	0	6.8	0
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(0.3)	0	(0.2)	0
Segment Reconciling Items \| Fees and other revenues
Segment Reporting Information [Line Items]
Revenues	174.9	151.8	516.8	434.8
Segment Reconciling Items \| Service businesses
Segment Reporting Information [Line Items]
Revenues	73.8	58.8	202.1	169.4
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	1	4.4	12.1	14.7
Segment Reconciling Items \| Investment income
Segment Reporting Information [Line Items]
Revenues	245.7	763.1	1,723.7	1,585.3
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	239.1	758.4	1,705.2	1,570.8
Segment Reconciling Items \| Interest expense
Segment Reporting Information [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	$ (54.2)	$ (56.4)	$ (167)	$ (160.8)

Segment Information Underwritin

Segment Information Underwriting Margins and Combined Ratios for our Underwriting Operations (Detail) - Underwriting operations	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Personal Lines \| Agency
Segment Reporting Information [Line Items]
Underwriting Margin	1.00%	14.90%	6.30%	14.90%
Combined Ratio	99.00%	85.10%	93.70%	85.10%
Personal Lines \| Direct
Segment Reporting Information [Line Items]
Underwriting Margin	(1.30%)	12.00%	3.50%	13.10%
Combined Ratio	101.30%	88.00%	96.50%	86.90%
Operating Segments
Segment Reporting Information [Line Items]
Underwriting Margin	(0.40%)	12.20%	4.40%	12.50%
Combined Ratio	100.40%	87.80%	95.60%	87.50%
Operating Segments \| Personal Lines
Segment Reporting Information [Line Items]
Underwriting Margin	(0.20%)	13.40%	4.90%	14.00%
Combined Ratio	100.20%	86.60%	95.10%	86.00%
Operating Segments \| Commercial Lines
Segment Reporting Information [Line Items]
Underwriting Margin	10.50%	12.80%	11.30%	12.70%
Combined Ratio	89.50%	87.20%	88.70%	87.30%
Operating Segments \| Property
Segment Reporting Information [Line Items]
Underwriting Margin	(42.30%)	(11.80%)	(25.10%)	(14.80%)
Combined Ratio	142.30%	111.80%	125.10%	114.80%
Combined ratio amortization points	0.027	0.032	0.028	0.033

Dividends (Details)

Dividends (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended												3 Months Ended		9 Months Ended
	Sep. 30, 2021	Aug. 31, 2021	Jul. 31, 2021	Apr. 30, 2021	Mar. 31, 2021	Jan. 31, 2021	Oct. 31, 2020	Sep. 30, 2020	Jul. 31, 2020	Apr. 30, 2020	Mar. 31, 2020	Jan. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Feb. 28, 2021	Dec. 31, 2020	Aug. 31, 2020	Jun. 30, 2020	Feb. 29, 2020	Dec. 31, 2019
Dividends payable on common shares	$ 58.5							$ 58.6					$ 58.5	$ 58.6	$ 58.5	$ 58.6			$ 2,694.5
Common Stock, Dividends, Per Share, Declared													$ 0.10	$ 0.10	$ 0.30	$ 0.30
Quarterly dividend [Member]
Common Stock, Dividends, Per Share, Cash Paid			$ 0.10	$ 0.10		$ 0.10	$ 0.10		$ 0.10	$ 0.10		$ 0.10
Common Stock, Dividends, Per Share, Declared		$ 0.10
Annual variable dividend
Common Stock, Dividends, Per Share, Cash Paid						$ 4.50						$ 2.25
Series B Preferred Stock
Preferred Stock, Dividends, Per Share, Cash Paid	$ 26.875				$ 26.875			$ 26.875			$ 26.875
Dividends payable on common shares		$ 13.4																$ 13.4		$ 13.4		$ 13.4
Common equities \| Quarterly dividend [Member]
Dividends payable on common shares	$ 58.5				$ 58.5			$ 58.6			$ 58.5		$ 58.5	$ 58.6	$ 58.5	$ 58.6	$ 58.5		58.6		$ 58.5		$ 58.5
Common equities \| Annual variable dividend
Dividends payable on common shares																			$ 2,635.9				$ 1,316.9

Other Comprehensive Income (L_3

Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
AOCI before Tax, Attributable to Parent, beginning balance	$ 619.6	$ 1,237.3	$ 1,187.4	$ 439.1
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment and Tax	(117.9)	163.3	(494.6)	1,447.7
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Gain (Loss), before Reclassification and Tax	(0.3)	0	(0.9)	0
OCI, before Reclassifications, before Tax, Attributable to Parent	(118.2)	163.3	(495.5)	1,447.7
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	46.4	194.3	237.5	681
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	(0.2)	(0.3)	(0.8)	(0.8)
Total reclassification adjustment for amounts realized in net income, pre tax	46.2	194	236.7	680.2
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	(164.4)	(30.7)	(732.2)	767.5
AOCI before Tax, Attributable to Parent, ending balance	455.2	1,206.6	455.2	1,206.6
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
AOCI Tax, Attributable to Parent, beginning balance	(136.4)	(262.3)	(255.7)	(94.7)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax	24.8	(34.4)	103.9	(304.1)
Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Tax, Portion Attributable to Parent	(0.1)	0	(0.2)	0
OCI, Debt Securities, Available-for-Sale, Gain (Loss), before Adjustment, Tax	24.9	(34.4)	104.1	(304.1)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	(9.7)	(40.8)	(49.9)	(143)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax	(0.1)	(0.1)	(0.2)	(0.2)
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	(9.6)	(40.7)	(49.7)	(142.8)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	34.5	6.3	153.8	(161.3)
AOCI Tax, Attributable to Parent, ending balance	(101.9)	(256)	(101.9)	(256)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	483.2	975	931.7	344.4
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	(93.1)	128.9	(390.7)	1,143.6
Foreign currency translation adjustment	(0.2)	0	(0.7)	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(93.3)	128.9	(391.4)	1,143.6
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	36.7	153.5	187.6	538
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	(0.1)	(0.2)	(0.6)	(0.6)
Total reclassification adjustment for amounts realized in net income, after tax	36.6	153.3	187	537.4
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(129.9)	(24.4)	(578.4)	606.2
After tax total accumulated other comprehensive income, ending balance	353.3	950.6	353.3	950.6
Total net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	498.8	991	947.3	360.8
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	(93.1)	128.9	(390.7)	1,143.6
Foreign currency translation adjustment	0	0	0	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(93.1)	128.9	(390.7)	1,143.6
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	36.7	153.5	187.6	538
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	0	0	0	0
Total reclassification adjustment for amounts realized in net income, after tax	36.7	153.5	187.6	538
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(129.8)	(24.6)	(578.3)	605.6
After tax total accumulated other comprehensive income, ending balance	369	966.4	369	966.4
Net unrealized gains (losses) on forecasted transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(15.1)	(16)	(15.6)	(16.4)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	0	0	0	0
Foreign currency translation adjustment	0	0	0	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	0	0	0
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	(0.1)	(0.2)	(0.6)	(0.6)
Total reclassification adjustment for amounts realized in net income, after tax	(0.1)	(0.2)	(0.6)	(0.6)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.1	0.2	0.6	0.6
After tax total accumulated other comprehensive income, ending balance	(15)	(15.8)	(15)	(15.8)
Accumulated Foreign Currency Adjustment Attributable to Parent
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(0.5)	0	0	0
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	0	0	0	0
Foreign currency translation adjustment	(0.2)	0	(0.7)	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(0.2)	0	(0.7)	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	0	0	0
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	0	0	0	0
Total reclassification adjustment for amounts realized in net income, after tax	0	0	0	0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(0.2)	0	(0.7)	0
After tax total accumulated other comprehensive income, ending balance	$ (0.7)	$ 0	$ (0.7)	$ 0

Other Comprehensive Income (L_4

Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement, Additional Information (Detail) $ in Millions	Sep. 30, 2021USD ($)
Statement of Comprehensive Income [Abstract]
Net unrealized losses on forecasted transactions, expected to reclassify into income within the next 12 months	$ 0.5

Goodwill and Intangible Asset_2

Goodwill and Intangible Assets - Net Carrying Amount of Other Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Schedule of Other Intangible Assets [Line Items]
Indefinite lived intangible assets	$ 12.4	$ 12.4	$ 12.4
Total	131.9	171.4	185.5
Accumulated amortization	369.2	326.1	312
Licensing Agreements
Schedule of Other Intangible Assets [Line Items]
Accumulated amortization	0.6	0.6	0.6
Finite-Lived Intangible Assets
Schedule of Other Intangible Assets [Line Items]
Intangible assets subject to amortization	119.5	159	173.1
Accumulated amortization	$ 368.6	$ 325.5	$ 311.4

Goodwill and Intangible Asset_3

Goodwill and Intangible Assets - Finite-lived Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	$ 369.2	$ 326.1	$ 312
Policies in force
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	256.2	256.2	256.2
Accumulated Amortization	237.9	210.4	201.3
Net Carrying Amount	18.3	45.8	54.9
Agency relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	159.2	159.2	159.2
Accumulated Amortization	73.9	65.4	62.6
Net Carrying Amount	85.3	93.8	96.6
Software rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	69.1	69.1	69.1
Accumulated Amortization	56.2	49.7	47.5
Net Carrying Amount	12.9	19.4	21.6
Trade name
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	3.6	0	0
Accumulated Amortization	0.6	0	0
Net Carrying Amount	3	0	0
Finite-Lived Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	488.1	484.5	484.5
Accumulated Amortization	368.6	325.5	311.4
Net Carrying Amount	$ 119.5	$ 159	$ 173.1

Goodwill and Intangible Asset_4

Goodwill and Intangible Assets Goodwill (Details) - USD ($)	Jun. 01, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Finite-Lived Intangible Assets [Line Items]
Goodwill, Acquired During Period				$ 0
Goodwill, Impaired, Accumulated Impairment Loss		$ 0		0
Amortization of intangible assets		14,600,000	$ 14,200,000	43,100,000	$ 42,800,000
Operating Segments \| Underwriting operations \| Property
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets		$ 14,200,000	$ 14,200,000	$ 42,500,000	$ 42,800,000
Trade name \| Protective Insurance
Finite-Lived Intangible Assets [Line Items]
Intangible assets acquired, related to trade name	$ 3,600,000
Finite-Lived Intangible Asset, Useful Life	2 years

Acquisition (Details)

Acquisition (Details) - Protective Insurance - USD ($)	Jun. 01, 2021	Sep. 30, 2021	Sep. 30, 2021
Business Combination, Separately Recognized Transactions [Line Items]
Price per common share	$ 23.30
Aggregate purchase amount	$ 338,000,000
Percent of company wide net premiums written		1.00%
Tax basis goodwill	0
Assets acquired	1,700,000,000
Investment securities acquired	1,100,000,000
Cash and cash equivalents, restricted cash and restricted cash equivalents acquired	24,300,000
Reinsurance recoverables acquired	452,400,000
Liabilities assumed in acquisition	1,400,000,000
Unearned premiums	66,900,000
Loss and loss adjustment expense reserves	1,100,000,000
Repayments of Lines of Credit	20,000,000
Revenue included in consolidated results		$ 144,300,000	$ 190,800,000
Net income included in consolidated results		$ 11,100,000	$ 13,400,000
Trade name
Business Combination, Separately Recognized Transactions [Line Items]
Intangible assets acquired, related to trade name	$ 3,600,000
Finite-Lived Intangible Asset, Useful Life	2 years